Specifying the Type of Computation

The most important keywords in a PSI3 input file are those which tell the program what type of computation are to be performed. They jobtype keyword tells the psi3 program whether this is a single-point computation, a geometry optimization, a vibrational frequency calculation, etc. The reference keyword specifies whether an RHF, ROHF, UHF, etc., reference is to be used for the SCF wavefunction. The wfn specifies what theoretical method is to be used, either SCF, determinant-based CI, coupled-cluster, etc. Also of critical importance are the charge and multiplicity of the molecule, the molecular geometry, and the basis set to be used. The latter two topics are discussed below in sections 3.3 and 3.7. General keywords determining the general type of computation to be performed are described below.

LABEL = string

This is a character string to be included in the output to help keep track of what computation has been run. It is not otherwise used by the program. There is no default.

JOBTYPE = string

This tells the program whether to run a single-point energy calculation (SP), a geometry optimization (OPT), a series of calculations at different displaced geometries (DISP), a frequency calculation (FREQ), frequencies only for symmetric vibrational modes (SYMM_FREQ), a Diagonal Born-Oppenheimer Correction (DBOC) energy computation, or certain response properties (RESPONSE). The default is SP.

WFN = string

This specifies the wavefunction type. Possible values are:
SCF, MP2, MP2R12, CIS, DETCI, CASSCF, RASSCF, CCSD, CCSD_T, BCCD, BCCD_T, EOM_CCSD, ZAPTN.

REFERENCE = string

This specifies the type of SCF calculation one wants to do. It can be one of RHF (for a closed shell singlet), ROHF (for a restricted open shell calculation), UHF (for an unrestricted open shell calculation), or TWOCON (for a two configuration singlet). The default is RHF.

MULTP = integer

Specifies the multiplicity of the molecule, i.e., 2S+1. Default is 1 (singlet).

CHARGE = integer

Specifies the charge of the molecule. Default is 0.

DERTYPE = string

This specifies the order of the derivative that is to be obtained. The default is NONE (energy only).

DOCC = integer vector

This gives the number of doubly occupied orbitals in each irreducible representation. There is no default. If this is not given, cscf will attempt to guess at the occupations.

SOCC = integer vector

This gives the number of singly occupied orbitals in each irreducible representation. There is no default. If this is not given, cscf will attempt to guess at the occupations.

FREEZE_CORE = string

PSI3 can automatically freeze core orbitals. Core orbitals are defined as follows:

 H-Be  no core 
 B-Ne  1s 
Na-Ar  small: 1s2s
       large: 1s2s2p

YES or TRUE will freeze the core orbitals, SMALL or LARGE are for elements Na-Ar. The default is NO or FALSE. Always check to make sure that the occupations are correct!