Electronic Energy at Fixed Nuclear Geometry
===========================================

The |molcas| suite of Quantum Chemical programs is modular in
design, and a desired calculation is achieved by executing a list of
|molcas| program modules in succession, occasionally manipulating
the program information files. If the information files from a previous
calculation are saved, then a subsequent calculation need not recompute
them. This is dependent on the correct information being preserved in
the information files for the subsequent calculations. Each module has keywords
to specify the functions to be carried out, and many modules rely on the
specification of keywords in previous modules.

In the present examples the calculations will be designed by preparing
a single file in which the input for the different programs is presented
sequentially. The initial problem will be to compute an electronic energy
at a fixed geometry of the nuclei, and this will be performed using different
methods and thus requiring different |molcas| program modules.

First, the proper |molcas| environment has to be set up which requires that
following variables must be properly defined, for instance: ::

  export MOLCAS=/home/molcas/molcas
  export Project=CH4
  export WorkDir=/home/user/tmp

If not defined, |molcas| provides default values for the above environment variables:

* The :variable:`MOLCAS` variable will be set to the latest implemented version of the code.

  This variable is set directly in the |molcas| home directory

* :variable:`Project` and :variable:`WorkDir` have the default values None and $PWD, respectively.

  It is very important that the molcas driver, called by command :command:`molcas`,
  and built during the installation of the code, is included in the $PATH.

The first run involves a calculation of the SCF energy of the methane
(:math:`\ce{CH4}`) molecule. Three programs should be used: :program:`GATEWAY` to specify
information about the system, :program:`SEWARD` to compute
and store the one- and two-electron integrals, and :program:`SCF` to obtain
the Hartree--Fock SCF wave function and energy.

.. compound::

  The three |molcas| programs to
  be used leads to three major entries in the input file: :program:`GATEWAY`, :program:`SEWARD`, and :program:`SCF`.
  The :program:`GATEWAY` program contains the nuclear geometry in cartesian
  coordinates and the label for the one-electron basis set.
  The keyword :kword:`coord` allows automatic insertion of :program:`GATEWAY` input from a standard
  file containing the cartesian coordinates in angstrom which can be generated by
  programs like :program:`LUSCUS` or :program:`MOLDEN`).
  No symmetry is being considered so the keyword :kword:`group=C1` is used to force the program not
  to look for symmetry in the :math:`\ce{CH4}` molecule, and ,thus, input for :program:`SEWARD` is not required.
  In closed-shell cases, like :math:`\ce{CH4}`, input for :program:`SCF` is not required. All the input
  files discussed here can be found at :file:`$MOLCAS/doc/samples/problem_based_tutorials`, including the file
  :file:`SCF.energy.CH4` described below.

  .. extractfile:: problem_based_tutorials/SCF.energy.CH4.input

    *SCF energy for CH4 at a fixed nuclear geometry.
    *File: SCF.energy.CH4
    *
    &GATEWAY
     Title = CH4 molecule
     coord = CH4.xyz
     basis = STO-3G
     group = C1

    &SEWARD
    &SCF

  where the content of the :file:`CH4.xyz` file is:

  .. extractfile:: problem_based_tutorials/CH4.xyz

    5
    distorted CH4 coordinates in angstroms
    C    0.000000     0.000000     0.000000
    H    0.000000     0.000000     1.050000
    H    1.037090     0.000000    -0.366667
    H   -0.542115    -0.938971    -0.383333
    H   -0.565685     0.979796    -0.400000

.. compound::

  To run |molcas|, simply execute the command ::

    molcas SCF.energy.CH4.input > SCF.energy.CH4.log 2 > SCF.energy.CH4.err

  where the main output is stored in file :file:`SCF.energy.CH4.log`

  or ::

    molcas -f SCF.energy.CH4.input

  where the main output is stored in :file:`SCF.energy.CH4.log`, and the default error file in :file:`SCF.energy.CH4.err`.

The most relevant information is contained in the output file, where the :program:`GATEWAY` program
information describing the nuclear geometry, molecular symmetry, and the data
regarding the one-electron basis sets and the calculation of one- and
two-electron integrals, as described in :numref:`TUT:sec:seward`. Next,
comes the output of program :program:`SCF` with information of the electronic
energy, wave function, and the Hartree--Fock (HF) molecular orbitals
(see :numref:`TUT:sec:scf`).

Files containing intermediate information, integrals, orbitals, etc, will be
kept in the $WorkDir directory for further use. For instance, files
:file:`$Project.OneInt` and :file:`$Project.OrdInt` contain the one- and
two-electron integrals stored in binary format. File :file:`$Project.ScfOrb`
stores the HF molecular orbitals in ASCII format, and
:file:`$Project.RunFile` is a communication file between programs. All these
files can be used later for more advanced calculations avoiding a
repeat of certain calculations.

There are graphical utilities that can be used for the analysis of the
results. By default, |molcas| generates files which can be read with the
:program:`MOLDEN` program and are found in the :file:`$WorkDir` including the file :file:`CH4.scf.molden`.
This file contains information about molecular geometry and molecular orbitals, and requires the use if *Density Mode* in :program:`MOLDEN`.
However, |molcas| has its own graphical tool, program :program:`LUSCUS`, which is a viewer based on openGL and allows the visualization of
molecular geometries, orbitals, densities, and density differences. For
example, a graphical display of the :math:`\ce{CH4}` molecule can be obtained from a standard coordinate file by the following command: ::

  luscus CH4.xyz

In order to obtain the information for displaying molecular orbitals and densities,
it is necessary to run the |molcas| program called :program:`GRID_IT`:

.. extractfile:: problem_based_tutorials/SCF.energy_grid.CH4.input

  *SCF energy for CH4 at a fixed nuclear geometry plus a grid for visualization.
  *File: SCF.energy_grid.CH4
  *
  &GATEWAY
   Title = CH4 molecule
   coord = CH4.xyz
   basis = STO-3G
   Group = C1

  &SEWARD; &SCF

  &GRID_IT
   All

Now, execute the |molcas| program: ::

  molcas SCF.energy_grid.CH4.input -f

.. compound::

  In the :file:`$WorkDir` and :file:`$PWD` directories a new file is generated, :file:`CH4.lus` which
  contains the information required by the :program:`GRID_IT` input. The file can
  be visualized by :program:`LUSCUS` (Open source program, which can be downloaded and
  installed to your Linux, Windows, or MacOS workstation or laptop). By typing the command: ::

    luscus CH4.lus

  a window will be opened displaying the molecule and its charge density. By proper
  selection of options with the mouse buttons, the shape and size of several molecular orbitals
  can be visualized.

:program:`GRID_IT` can also be run separately, if an orbital file is specified in
the input, and the :file:`$WorkDir` directory is available.

More information can be found in :numref:`UG:sec:gridit`.

As an alternative to running a specific project, the short script provided below can be placed
in the directory :file:`$MOLCAS/doc/samples/problem_based_tutorials` with the name :file:`project.sh`.
Simply execute the shell script, :command:`project.sh $Project`, where :command:`$Project` is the |molcas| input,
and output files, error files, and a :file:`$WorkDir` directory called :file:`$Project.work` will be obtained.

.. extractfile:: problem_based_tutorials/project.sh

  #!/bin/bash

  export MOLCAS=$PWD
  export MOLCAS_DISK=2000
  export MOLCAS_MEM=64
  export MOLCAS_PRINT=3

  export Project=$1
  export HomeDir=$MOLCAS/doc/samples/problem_based_tutorials
  export WorkDir=$HomeDir/$Project.work
  mkdir $WorkDir 2>/dev/null
  molcas $HomeDir/$1 >$HomeDir/$Project.log 2>$HomeDir/$Project.err
  exit

In order to run a Kohn--Sham density functional calculation, |molcas| uses the
same :program:`SCF` module, and, therefore, the only change needed are the specification
of the DFT option and required functional (e.g. B3LYP) in the :program:`SCF` input:

.. extractfile:: problem_based_tutorials/DFT.energy.CH4.input

  *DFT energy for CH4 at a fixed nuclear geometry plus a grid for visualization.
  *File: DFT.energy.CH4
  *
  &GATEWAY
   Title = CH4 molecule
   coord = CH4.xyz
   basis = STO-3G
   group = C1
  &SEWARD
  &SCF
   KSDFT = B3LYP
  &GRID_IT
   All

Similar graphical files can be found in :file:`$WorkDir` and :file:`$PWD`.

The next step is to obtain the second-order Møller--Plesset perturbation (MP2)
energy for methane at the same molecular geometry using the same one-electron
basis set. Program :program:`MBPT2` is now used, and it is possible to take
advantage of having previously computed the proper integrals with :program:`SEWARD`
and the reference closed-shell HF wave function with the :program:`SCF` program.
In such cases, it is possible to keep the same definitions as before and simply prepare a file
containing the :program:`MBPT2` input and run it using the :command:`molcas`
command.

The proper intermediate file will be already in :file:`$WorkDir`.
On the other hand, one has to start from scratch, all required inputs should
be placed sequentially in the :file:`MP2.energy.CH4` file.
If the decision is to start the project from the beginning, it is probably a good idea to remove
the entire :file:`$WorkDir` directory, unless it is known for certain the exact nature of the files contained in this directory.

.. extractfile:: problem_based_tutorials/MP2.energy.CH4.input

  *MP2 energy for CH4 at a fixed nuclear geometry.
  *File: MP2.energy.CH4
  *
  &GATEWAY
   Title = CH4 molecule
   coord = CH4.xyz
   basis = STO-3G
   group = C1
  &SEWARD
  &SCF
  &MBPT2
   Frozen = 1

In addition to the calculation of a HF wave function, an MP2 calculation has been performed with
a frozen deepest orbital, the carbon 1s, of :math:`\ce{CH4}`. Information about the output
of the :program:`MBPT2` program can be found on :numref:`TUT:sec:mbpt2`.

.. compound::

  The :program:`SCF` program works by default with closed-shell systems with an
  even number of electrons at the Restricted Hartee--Fock (RHF) level. If,
  instead there is a need to use the Unrestricted Hartree--Fock (UHF) method, this can be schieved by invoking the
  keyword :kword:`UHF`. This is possible for both even and odd electron systems.
  For instance, in a system with an odd number of electrons such as the :math:`\ce{CH3}` radical, with the
  following Cartesian coordinates

  .. extractfile:: problem_based_tutorials/CH3.xyz

    4
    CH3 coordinates in angstrom
    C    0.000000     0.000000     0.000000
    H    0.000000     0.000000     1.050000
    H    1.037090     0.000000    -0.366667
    H   -0.542115    -0.938971    -0.383333

  the input to run an open-shell UHF calculation is easily obtained:

.. extractfile:: problem_based_tutorials/SCF.energy_UHF.CH3.input

  *SCF/UHF energy for CH3 at a fixed nuclear geometry
  *File: SCF.energy_UHF.CH3
  *
  &GATEWAY
   Title = CH3 molecule
   coord = CH3.xyz
   basis = STO-3G
   group = C1
  &SEWARD
  &SCF
   UHF

If the system is charged, this must be indicated in the
:program:`SCF` input, for example, by computing the cation of the :math:`\ce{CH4}` molecule
at the UHF level:

.. extractfile:: problem_based_tutorials/SCF.energy_UHF.CH4plus.input

  *SCF/UHF energy for CH4+ at a fixed nuclear geometry
  *File: SCF.energy_UHF.CH4plus
  *
  &GATEWAY
   Title = CH4+ molecule
   coord = CH4.xyz
   basis = STO-3G
   group = c1
  &SEWARD
  &SCF
   UHF
   Charge = +1

The Kohn--Sham DFT calculation can be also run using the UHF algorithm:

.. extractfile:: problem_based_tutorials/DFT.energy.CH4plus.input

  *DFT/UHF energy for CH4+ at a fixed nuclear geometry
  *File: DFT.energy.CH4plus
  *
  &GATEWAY
   Title = CH4+ molecule
   coord = CH4.xyz
   basis = STO-3G
   group = C1
  &SEWARD
  &SCF
   KSDFT = B3LYP
   UHF
   Charge = +1

For the UHF and UHF/DFT methods it is also possible to specify
:math:`\alpha` and :math:`\beta` orbital occupations in two ways.

#. First, the keyword :kword:`ZSPIn` can be invoked in the :program:`SCF` program, which represents the
   difference between the number of :math:`\alpha` and :math:`\beta` electrons.

   For example, setting the keyword to 2 forces the program to converge to a result with two more :math:`\alpha` than :math:`\beta` electrons.

   .. extractfile:: problem_based_tutorials/DFT.energy_zspin.CH4.input

     *DFT/UHF energy for different electronic occupation in CH4 at a fixed nuclear geometry
     *File: DFT.energy_zspin.CH4
     *
     &GATEWAY
      Title = CH4 molecule
      coord = CH4.xyz
      basis = STO-3G
      group = c1
     &SEWARD
     &SCF
      Title = CH4 molecule zspin 2
      UHF; ZSPIN = 2
      KSDFT = B3LYP

   The final occupations in the output will show six :math:`\alpha` and four :math:`\beta` orbitals.

#. Alternatively, instead of :kword:`ZSPIn`, it is possible to specify
   occupation numbers with keyword :kword:`Occupation` at the beginning of the SCF calculation.

   This requires an additional input line containing the occupied :math:`\alpha` orbitals (e.g. 6 in this case), and a second line
   with the :math:`\beta` orbitals (e.g. 4 in this case). Sometimes, SCF convergence may be improved by using this option.

Different sets of methods use other |molcas| modules. For example, to perform a Complete
Active Space (CAS) SCF calculation, the :program:`RASSCF` program has to be used. This
module requires starting trial orbitals, which can be obtained from a previous SCF
calculation or, automatically, from the :program:`SEWARD` program which provides trial orbitals by
using a model Fock operator.

Recommended keywords are

* :kword:`Nactel` defines the total number of active
  electrons, holes in Ras1, and particles in Ras3, respectively. The last two values
  are only for RASSCF-type calculations.
* :kword:`Inactive` indicates the number of inactive orbitals where the occupation is always 2 in the CASSCF reference, and
* :kword:`Ras2` defines the number of active orbitals.

By default, the wave function for the lowest state corresponds to the symmetry with spin multiplicity of 1.
Most of the input may not be necessary, if one has prepared and linked an INPORB file with the different orbital types defined by
a program like :program:`LUSCUS`.

.. extractfile:: problem_based_tutorials/CASSCF.energy.CH4.input

  *CASSCF energy for CH4 at a fixed nuclear geometry
  *File: CASSCF.energy.CH4
  *
  &GATEWAY
   coord = CH4.xyz
   basis = STO-3G
   group = C1
  &SEWARD
  &RASSCF
   Title = CH4 molecule
   Spin = 1; Nactel = 8 0 0; Inactive = 1; Ras2 = 8
  &GRID_IT
   All

In this case, the lowest singlet state (i.e. the ground dstate) is computed, since this is a
closed-shell situation with an active space of eight electrons in eight orbitals and
with an inactive C 1s orbital, the lowest orbital of the :math:`CH4` molecule. This is a CASSCF example in which all the valence
orbitals and electrons (C 2s, C 2p and 4 |x| H 1s) are included
in the active space and allows complete dissociation into
atoms. If this is not the goal, then the three almost degenerate
highest energy occupied orbitals and the corresponding antibonding unoccupied orbitalsmust be active, leading to
a 6 in 6 active space.

Using the CASSCF wave function as a reference, it is possible to perform a second-order
perturbative, CASPT2, correction to the electronic energy by employing the
:program:`CASPT2` program. If all previously calculated files are retained in the
:file:`$WorkDir` directory, in particular, integral files (:file:`CH4.OneInt`, :file:`CH4.OrdInt`),
the CASSCF wave function information file (:file:`CH4.JobIph`), and communication file :file:`CH4.RunFile`), it will not be
necessary to re-run programs :program:`SEWARD`, and :program:`RASSCF`. In this case
case, it is enough to prepare a file containing input only for the :program:`CASPT2` program followed be execution.
Here, however, for the sake of completness, input to all |molcas| modules is provided:

.. extractfile:: problem_based_tutorials/CASPT2.energy.CH4.input

  *CASPT2 energy for CH4 at a fixed nuclear geometry
  *File: CASPT2.energy.CH4
  *
  &GATEWAY
   coord = CH4.xyz; basis = STO-3G; group = C1
  &SEWARD
  &RASSCF
     LumOrb
     Spin = 1; Nactel = 8 0 0; Inactive = 1; Ras2 = 8
  &CASPT2
   Multistate = 1 1

In most of cases, the Hartree--Fock orbitals will be a better choice as starting orbitals.
In that case, the :program:`RASSCF` input has to include keyword :kword:`LumOrb` to read
from any external source of orbitals other than those generated by the :program:`SEWARD` program.
By modifying input to the :program:`SCF` program, it is possible to generate
alternative trial orbitals for the :program:`RASSCF` program. Since a new set of trial orbitals is used,
the input to the :program:`RASSCF` program is also changed. Now, the number of
active orbitals, as well as the number of active electrons, are 6.

The two lowest orbitals (:kword:`Inactive` 2) are excluded from the active space
and one other orbital is placed in the secondary space.
If the previous (8,8) full valence space was used,
the :program:`CASPT2` program would not be able to include more electronic correlation energy,
considering that the calculation involves a minimal basis set.
The input for the :program:`CASPT2` program includes a frozen C 1s orbital, the lowest orbital
in the :math:`\ce{CH4}` molecule.

The charge and multiplicity of our wave function can be changed by computing the
:math:`\ce{CH4^+}` cation with the same methods. The :program:`RASSCF` program defines
the character of the problem by specifying the number of electrons, the spin multiplicity, and the spatial
symmetry. In the example below, there is one less electron giving rise to doublet multiplicity:

.. extractfile:: problem_based_tutorials/CASSCF.energy.CH4plus.input

  *CASSCF energy for CH4+ at a fixed nuclear geometry
  *File: CASSCF.energy.CH4plus
  *
  &GATEWAY
   Title = CH4+ molecule
   coord = CH4.xyz; basis = STO-3G; Group = C1
  &SEWARD
  &RASSCF
   Spin = 2; Nactel = 7 0 0; Inactive = 1; Ras2 = 8

No further modification is needed to the :program:`CASPT2` input:

.. extractfile:: problem_based_tutorials/CASPT2.energy.CH4plus.input

  *CASPT2 energy for CH4+ at a fixed nuclear geometry
  *File: CASPT2.energy.CH4plus
  *
  &GATEWAY
   coord = CH4.xyz; basis = STO-3G; group = C1
  &SEWARD
  &RASSCF
   Title = CH4+ molecule
   Spin = 2; Nactel = 1 0 0; Inactive = 4; Ras2 = 1
  &CASPT2

A somewhat more sophisticated calculation can be performed at the
Restricted Active Space (RAS) SCF level. In such a situation, the level of excitation
in the CI expansion can be controlled by restricting the number of holes
and particles present in certain orbitals.

.. extractfile:: problem_based_tutorials/RASSCF.energy.CH4.input

  *RASSCF energy for CH4 at a fixed nuclear geometry
  *File: RASSCF.energy.CH4
  *
  &GATEWAY
   coord = CH4.xyz; basis = STO-3G; group = C1
  &SEWARD
  &RASSCF
   Title = CH4 molecule
   Spin = 1; Nactel = 8 1 1
   Inactive = 1; Ras1 = 1; Ras2 = 6; Ras3 = 1

In particular, the previous calculation includes one orbital within the Ras1
space and one orbital within the Ras3 space. One hole (single excitation) at
maximum is allowed from Ras1 to Ras2 or Ras3, while a maximum of one particle
is allowed in Ras3, derived from either Ras1 or Ras2. Within Ras2, all types
of orbital occupations are allowed. The RASSCF wave functions can be used
as reference for multiconfigurational perturbation theory (RASPT2), but
this approach has not been as extensively tested as CASPT2, and, so experience is
still somewhat limited.

|molcas| also has the possibility of computing electronic energies at
different CI levels by using the :program:`MRCI` program. The input provided below involves
a Singles and Doubles Configuration Interaction (SDCI) calculation on the :math:`\ce{CH4}` molecule.
To set up the calculations, program :program:`MOTRA` which transforms
the integrals to molecular basis, and program :program:`GUGA` which computes the
coupling coefficients, must be run before the :program:`MRCI` program.
In :program:`MOTRA` the reference orbitals are specifiedi, and those employed
here are from an HF :program:`SCF` calculation including frozen orbitals. In :program:`GUGA`
the reference for the CI calculation is described by the number of correlated electrons,
the spatial and spin symmetry, the inactive orbitals always occupation 2 in
the reference space, and the type of CI expansion.

.. extractfile:: problem_based_tutorials/SDCI.energy.CH4.input

  *SDCI energy for CH4 at a fixed nuclear geometry
  *File: SDCI.energy.CH4
  *
  &GATEWAY
   coord = CH4.xyz; basis = STO-3G; group = c1
  &SEWARD
  &SCF
  &MOTRA
   Lumorb
   Frozen= 1
  &GUGA
   Electrons = 8
   Spin = 1
   Inactive= 4
   Active= 0
   Ciall= 1
  &MRCI
   SDCI

To use reference orbitals from a previous CASSCF calculation, the
:program:`RASSCF` program will have to be run before the :program:`MOTRA`
module. Also, if the spatial or spin symmetry are changed for the CI
calculation, the modifications will be introduced in the input to :program:`GUGA` program.
Many alternatives are possible for performing an MRCI calculation as shown in the next example below,
in which the reference space to perform the CI is multiconfigurational:

.. extractfile:: problem_based_tutorials/MRCI.energy.CH4.input

  *MRCI energy for CH4 at a fixed nuclear geometry
  *File: MRCI.energy.CH4
  *
  &GATEWAY
   Title = CH4 molecule
   coord = CH4.xyz; basis = STO-3G; group = c1
  &SEWARD
  &SCF
  &RASSCF
   LumOrb
   Spin= 1; Nactel= 6 0 0; Inactive= 2; Ras2= 6
  &MOTRA
   Lumorb
   Frozen= 1
  &GUGA
   Electrons= 8
   Spin= 1
   Inactive= 2
   Active= 3
   Ciall= 1
  &MRCI
   SDCI

The :program:`MRCI` program also allows the calculation of electronic energies using the
ACPF method. Another |molcas| program, :program:`CPF`, offers the possibility to
use the CPF, MCPF, and ACPF methods with a single reference function. The
required input is quite similar to that for the :program:`MRCI` program:

.. extractfile:: problem_based_tutorials/CPF.energy.CH4.input

  *CPF energy for CH4 at a fixed nuclear geometry
  *File: CPF.energy.CH4
  *
  &GATEWAY
   Title= CH4 molecule
   coord = CH4.xyz; basis = STO-3G; group = c1
  &SEWARD
  &SCF
  &MOTRA
   Lumorb
   Frozen= 1
  &GUGA
   Electrons= 8
   Spin = 1
   Inactive = 4
   Active = 0
   Ciall= 1
  &CPF
   CPF
  End Of Input

Finally, |molcas| can also perform closed- and open-shell coupled cluster
calculations at the CCSD and CCSD(T) levels. These calculations are controlled by
the :program:`CCSDT` program, whose main requirement is that the reference
function has to be generated with the :program:`RASSCF` program. The following input is
required to obtain a CCSD(T) energy for the :math:`\ce{CH4}` molecule:

.. extractfile:: problem_based_tutorials/CCSDT.energy.CH4.input

  *CCSDT energy for CH4 at a fixed nuclear geometry
  *File: CCSDT.energy.CH4
  *
  &GATEWAY
   Title= CH4 molecule
   coord = CH4.xyz; basis = STO-3G; group = c1
  &SEWARD
  &RASSCF
   Spin= 1; Nactel= 0 0 0; Inactive= 5; Ras2= 0
   OutOrbitals
   Canonical
  &MOTRA
   JobIph
   Frozen= 1
  &CCSDT
   CCT

Since this is a closed-shell calculation, the :program:`RASSCF` input
computes a simple RHF wave function with zero active electrons and orbitals using
keywords :kword:`OutOrbitals` and :kword:`Canonical`. The :program:`MOTRA` must
include the keyword :kword:`JobIph` to extract the wave function information
from file :file:`JOBIPH` which is automatically generated by :program:`RASSCF`. Finally,
the keywork :kword:`CCT` in program :program:`CCSDT` leads to the calculation of the
CCSD(T) energy using the default algorithms.

The :program:`CCSDT` program in |molcas| is especially suited to compute open-shell
cases. The input required to obtain the electronic energy of the :math:`\ce{CH4^+}` cation
with the CCSD(T) method is:

.. extractfile:: problem_based_tutorials/CCSDT.energy.CH4plus.input

  *CCSDT energy for CH4+ at a fixed nuclear geometry
  *File: CCSDT.energy.CH4plus
  *
  &GATEWAY
   Title= CH4+ molecule
   coord = CH4.xyz; basis = STO-3G; group = c1
  &SEWARD
  &RASSCF
   Spin= 2; Nactel= 1 0 0; Inactive= 4; Ras2= 1
   OutOrbitals
   Canonical
  &MOTRA
   JobIph
   Frozen= 1
  &CCSDT
   CCT

where the :program:`RASSCF` program generated the proper Restricted Open-Shell
Hartree--Fock (ROHF) reference. Different levels of spin adaptation are also available.

If solvent effects are desired, |molcas| includes two
models: Kirkwood and PCM. Adding a solvent effect to a ground state at HF, DFT, or CASSCF levels,
simply requires the inclusion of the keyword :kword:`RF-input` within the input for the :program:`SEWARD`
which calculates a self-consistend reaction field.

.. extractfile:: problem_based_tutorials/DFT.energy_solvent.CH4.input

  *DFT energy for CH4 in water at a fixed nuclear geometry
  *File: DFT.energy_solvent.CH4
  *
  &GATEWAY
   Title= CH4 molecule
   coord = CH4.xyz; basis = STO-3G; group = c1
   RF-input
     PCM-model; solvent= water
   End of RF-input
  &SEWARD
  &SCF
  KSDFT= B3LYP

Other programs such as :program:`CASPT2`, :program:`RASSI`, and :program:`MOTRA` require that
the reaction field is included as a perturbation with keyword :kword:`RFPErturbation`.
In the next example the correction is added at both the CASSCF and CASPT2 levels.

.. extractfile:: problem_based_tutorials/CASPT2.energy_solvent.CH4.input

  *CASPT2 energy for CH4 in acetone at a fixed nuclear geometry
  *File: CASPT2.energy_solvent.CH4
  *
  &GATEWAY
   Title= CH4 molecule
   coord = CH4.xyz; basis = STO-3G; group = c1
    RF-input
     PCM-model; solvent= acetone; AAre= 0.2
    End of RF-input
  &SEWARD
  &RASSCF
    Spin= 1; Nactel= 6 0 0; Inactive= 2; Ras2= 6
  &CASPT2
   Frozen= 1
   Multistate= 1 1
   RFPert

Notice that the tesserae of the average area in the PCM model (keyword
has been changed to the value required for acetone by the keyword :kword:`Aare`,
while the default is 0.4 Å:math:`^2` for water
(see :numref:`UG:sec:rfield`).
More detailed examples can be found in :numref:`TUT:sec:cavity`.
