8.11 Selecting orbitals and density matrices (ORBITAL, DENSITY)

As outlined in section 4.3, the information for each SCF or MCSCF calculation is stored in a dump record. Dump records contain orbitals, density matrices, orbital energies, occupation numbers, fock matrices and other information as wavefunction symmetries etc. Subsequent calculation can access the orbitals and density matrices from a particular record using the ORBITAL and DENSITY directives, respectively. These input cards have the same structure in all programs. The general format of the ORBITAL and DENSITY directives is as follows.

ORBITAL[,[RECORD=]record] [,[TYPE=]orbtype] [,STATE=state] [,SYM[METRY]=symmetry]
[,SPIN=spin] [,MS2=ms2] [,[N]ELEC=nelec] [,SET=iset] [,OVL] [,NOCHECK] [,IGNORE[_ERROR]]

DENSITY[,[RECORD=]record] [,[TYPE=]dentype] [,STATE=state] [,SYM[METRY]=symmetry]
[,SPIN=spin] [,MS2=ms2] [,[N]ELEC=nelec] [,SET=iset]

where the (optional) specifications can be used to select specific orbitals, if several different orbital sets are stored in the same record. The meaning of the individual specifications are as follows:

orbtype

Orbital type. This can be one of
CANONICAL: canonical or pseudo-canonical orbitals;
NATURAL: natural orbitals;
LOCAL: localized orbitals;
LOCAL(PM): localized Pipek-Mezey orbitals;
LOCAL(BOYS): localized Boys orbitals;
PROJECTED: projected virtual orbitals used in local calculations.
Without further specification, the most recently computed orbitals of the specified type are used. If the orbital type is not specified, the program will try to find the most suitable orbitals automatically. For instance, in MRCI calculations NATURAL orbitals will be used preferentially if available; MRPT (CASPT2) calculations will first search for CANONICAL orbitals, and local calculations will first look for LOCAL orbitals. Therefore, in most cases the orbital type needs not to be specified.

state

Specifies a particular state in the form $istate.isym$. For instance, 2.1 refers to the second state in symmetry 1. This can be used if density matrices or natural orbitals have been computed for different states in a state-averaged CASSCF calculation. If not given, the state-averaged orbitals are used. The specification of $isym$ is optional; it can also be defined using the SYMMETRY key.

dentype

Density type. This can be one of
CHARGE: charge density;
SPIN: UHF spin density;
TRANSITION: transition density matrix;
The default is CHARGE.

symmetry

Specifies a particular state symmetry. Alternatively, the state symmetry can be specified using STATE (see above).

spin

Spin quantum number, i.e. 0 for singlet, 1/2 for doublet, 1 for triplet, etc. Alternatively MS2 can be used.

ms2

$2 M_S$, i.e. 0 for singlet, 1 for doublet, 2 for triplet etc. Alternatively, SPIN can be used.

nelec

Number of electrons.

iset

Set number of orbitals. The orbital sets are numbered in the order they are stored.

If OVL is specified, the starting orbitals are obtained by maximizing the overlap with previous orbitals. By default, this is used if the basis dimension of the previous orbitals is different then the current one. If OVL is specified this procedure is used even if the basis dimensions are the same, which is occasionally useful if the contraction scheme changed.

If NOCHECK is specified, some consistency checks for finding correct orbitals are skipped, and error messages like "ORBITALS CORRESPOND TO DIFFERENT GEOMETRY" are ignored.

If IGNORE_ERROR is specified, MPn or triples calculations can be forced with other than canonical orbitals. Note that this can lead to meaningless results!

If any of the above options are given, they must be obeyed strictly, i.e., the program aborts if the request cannot be fulfilled.

Examples:

ORBITAL,2100.2                    !Use SCF orbitals
ORBITAL,2140.2                    !Use (state-averaged) MCSCF orbitals
ORBITAL,2140.2,CANONICAL          !use canonical MCSCF orbitals
ORBITAL,2140.2,NATURAL,STATE=2.1  !use natural MCSCF orbitals for second state in sym. 1