More Advanced Input Options

If you wish to add comments to your input file, you can start any line with % and the line will be a comment line. This can make the input file easier to understand because you can provide explainations about each keyword. Another way to make the input file more organized is to seperate it into sections that correspond to particular modules the calculation will use. This can be particularly helpful for more complicated computations which can utilize many of keywords. In the example below, a CCSD(T) computation for the BH molecule is performed using a cc-pVDZ basis set. The keywords are divided into sections and several new keywords are introduced, including ones to specify symmetry and orbital occupations. Orbitial occupations are specified by a list of integers enclosed in parentheses. These integers give the number of orbitials which belong to each irreducible representation in the point group. The ordering of the irreps are those given by Cotton in Chemical Applications of Group Theory. In this example, comment lines will be included to explain the new keywords used.

psi: (
  wfn = ccsd_t
  reference = rhf
)

default: (
  label = "BH cc-pVDZ CCSD(T)"

% Allocating memory for the calculation
  memory = (600.0 MB)

% charge and multiplicity (2S+1) default to values of 0 and 1, respectively
  charge = 0
  multp = 1

% The program will generally guess the symmetry of the molecule, but
% it can be overridden.  Here we specify C2V because only D2H and its
% subgroups can be used by the program.
  symmetry = c2v

% Number of doubly-occupied orbitals per irrep can be specified manually
% if desired
  docc = (3 0 0 0)

% Freeze the 1A1 orbital (Boron 1s-like) in the CCSD(T) computation
  frozen_docc = (1 0 0 0)
)

% The input section contains information about the molecule and the basis
% set.  The geometry here is specified by cartesian coordinates.
input: (
  basis = "cc-pVDZ"
  units = angstroms
  geometry = (
    ( b      0.0000        0.0000        0.0000)
    ( h      0.0000        0.0000        0.8000)
      )
  origin = (0.0 0.0 0.0)
)
% The modular input structure lets you specify convergence criteria for
% each part of the computation separately
scf: (
  maxiter = 100
  convergence = 11
)

The final example of this tutorial demonstrates an example of a complete-active-space self-consistent-field (CASSCF) computation. CAS computations require specification of several additional keywords because you must specify which orbitals you wish to be in the active space. The notation and ordering for specifying CAS orbitals is the same as for occupied orbitals.

% 6-31G** H2O Test CASSCF Energy Point

psi: (
  label = "6-31G** CASSCF H2O"
  jobtype = sp
  wfn = casscf
  reference = rhf
% The restricted_docc orbitals are those which are optimized, but are not
% in the active space.
  restricted_docc = (1 0 0 0)

% The active space orbitals; here, the valence orbitals are chosen
  active          = (3 0 1 2)

  basis = "6-31G**"
  zmat = (
    o
    h 1 1.00
    h 1 1.00 2 103.1
  )
)