Guess

Options

Algorithm-Related Options

Harris

Diagonalize the Harris functional [Harris85] for the initial guess. This is the default for all HF and DFT calculations.

Huckel

Requests that a Huckel guess be generated. This is the default for CNDO, INDO, MNDO, and MINDO3. The Huckel guess should be considered for PM6 calculations involving many second-row atoms.

RdScale

Read in the scale factor on atomic hardnesses used in iterative extended Huckel. The default is 7.0 times the QEq value.

OldHuckel

Use the old Huckel guess (pre-Gaussian 03) instead of CNDO or the updated Huckel.

INDO

Use the Gaussian 16 default guess: INDO for first-row systems and Huckel for second-row and beyond.

AM1

Do an AM1 calculation for the initial guess (currently only works with sparse matrix code). Guess=(AM1,Always) causes later steps in a geometry optimization to generate a new guess at each point and compare the energies with the density from the old point and the new guess and take the best one.

Core

Requests that the core Hamiltonian be diagonalized to form the initial guess. This is the default for AM1, PM3, PM3MM, PM6, and PDDG.

Orbital-Related Options

Permute

Read in a permutation of orbitals in the initial guess. The numbers of the generated guess orbitals are given in the order in which they should be used in the SCF. Ranges (e.g. 7–12) can be used, and all orbitals not listed are put in after the listed orbitals in their original order. Separate permutation lists for α and β orbitals must be specified (each list separated by a blank line) for open shell systems.

Alter

Indicates that the orbitals selected for occupation in the Hartree-Fock wavefunction should not be those of lowest energy. Normally, the occupied orbitals are selected as those with lowest eigenvalues for the one-electron Hamiltonian used in the initial guess programs. The alteration sections consist of a set of transpositions indicating that one of these occupied orbitals is to be replaced by one of the other (virtual) orbitals. Each such transposition is on a separate line and has two integers N₁ and N₂ (free format, separated by spaces or a comma as usual) indicating that orbital N₁ is to be swapped with orbital N₂. The list of orbital transpositions is terminated by the blank line at the end of the input section.

For UHF calculations, two such orbital alteration sections are required, the first specifying transpositions of α orbitals and the second specifying transpositions of β orbitals. Both sections are always required. Thus, even if only α transpositions are needed, the β section is required even though it is empty (and vice-versa). The second blank line to indicate an empty β section must be included.

Mix

Requests that the HOMO and LUMO be mixed so as to destroy α-β and spatial symmetries. This is useful in producing UHF wavefunctions for singlet states. Mixing orbitals is only done by default when generating a complex initial guess. NoMix says to not mix orbitals.

DensityMix[=N]

Whether to mix occupied and virtual orbital contributions in forming the initial guess density. N defaults to -3 (use Huckel eigenvalues to decide which orbitals to mix).

CopyChk

Copy the MOs and density unedited, unprojected and unchecked from the checkpoint file for the guess. Useful when importing from matrix element files.

Biorthogonalize

For an unrestricted guess, biorthogonalize the α and β MOs to maximally pair electrons of opposite spin. Done automatically when a UHF wavefunction is read-in as a guess for ROHF. This option is also useful in the combination of Guess=(BiOrth,Read,Only,Save) to replace the canonical UHF orbitals with ones which have alpha and beta orbitals matched up as much as possible for viewing in GaussView or other visualization packages.

NaturalOrbitals

Include natural orbitals in the checkpoint file. This must be accomplished via a separate job step specifying this option as well as Only and Save. See the discussion of the Population keyword for details.

Procedural Options

Only

Guess=Only functions as a calculation type keyword and requests that the calculation terminate once the initial guess is computed and printed. Note that the amount of orbital information that is printed is controlled by the Pop keyword. Guess=Only may not be used with the MOPAC-based semi-empirical methods (INDO, CNDO, MNDO, MINDO3).

This option is useful in preliminary runs to check if configuration alteration is necessary. For example, Guess=Only may be specified with CASSCF in order to obtain information on the number of CI configurations in the CAS active space (as well as the initial orbitals).

Guess=(Only,Read) may also be used to produce population and other post-calculation analyses from the data in a checkpoint file. For example, these options alone will produce a population analysis using the wavefunction in the checkpoint file. Guess(Only,Read) Prop will cause electrostatic properties to be calculated using the wavefunction in the checkpoint file.

Always

Requests that a new initial guess be generated at each point of an optimization. By default, the SCF results from the last point are used for the guess at the next point.

Fragment=N

Generate a guess built from fragment guesses or SCF solutions. The guess is saved on the checkpoint file regardless of if it is used in the current calculation, allowing it to be used in subsequent ones. This option is usually combined with Guess=Only to generate a guess from fragment guess orbitals (otherwise a full SCF calculation is performed for each fragment). Assignment of atoms to fragments and fragment charges and multiplicities are specified as described in Molecule Specifications, except that a negative spin multiplicity means that the unpaired orbitals for the fragment are to become β spin orbitals in the combined set.

Local

Requests that orbitals be localized using the Boys method [Boys60]. Occupied and virtual orbitals are localized separately, and the irreducible representations (after possible merging using LowSymm or NoSymm) are not mixed. Localized orbital analysis of a converged SCF wavefunction may then be done using a second job step, which includes Guess(Read,Local,Only) and Pop=Full in its route section.

Sparse

Perform a sparse SE calculation for the initial guess. This option may be useful for very, very large HF or DFT calculations using the sparse matrix facility.

Extra

Do an extra, new initial guess when reading orbitals from the RWF (i.e., during geometry optimizations). By default, this is done if the default Harris guess is allowed, no alteration of configuration was requested, and the optimization did not take a small step as flagged by variable 4 in ILSW. Use NoExtra to disable this feature.

Fock

Reuse Fock matrices rather than orbitals when reading from previous results from the read-write or checkpoint files. This is the default for periodic boundary conditions calculations if Guess=Alter is not specified. NoFock disables this behavior, and it is the default for non-PBC calculations.

Guess Reading/Saving Options

Read

Requests that the initial guess be read from the checkpoint file (Guess=Read is often specified along with Geom=Checkpoint). This option may be combined with Alter, in which case the orbitals are read from the checkpoint file, projected onto the current basis set, and then the specified alterations are made. Checkpoint is a synonym for Read.

TCheck

Read SCF restart information from the checkpoint file if possible; otherwise, read the initial guess. Designed for multistep jobs so that the SCF can be restarted in a later step when possible or it can be run with Guess=Read otherwise. This option implies CIS=Read or TD=Read when applicable/appropriate.

Alpha

Use alpha orbitals for both alpha and beta guess during Guess=Read.

Translate

Translate requests that the coordinates of the atoms used to produce a guess, which is read in, be translated to the current atomic coordinates. This is the default. It may fail in unusual cases, such as when a wavefunction is used as a guess for a system with a different stoichiometry, in which case Guess=NoTranslate should be specified.

Cards

Specifies that after the initial guess is generated, some or all of the orbitals will be replaced with ones read from the input stream. This option can be used to read a complete initial guess from the input stream by replacing every orbital. The replacement orbitals are placed in the input section following the guess alteration commands, if any. For UHF, there are separate α and β replacement orbital input sections.

The replacement orbitals input section (the α replacement orbitals section for UHF) begins with a line specifying the Fortran format with which to read the replacement orbital input, enclosed in parentheses. For example: (4E20.8). The remainder of the section contains one or more instances of the following:

IVec               Orbital to replace (0=end, -1=replace all orbitals in order).
(A(I,IVec),I=1,N)    New orbital in the format specified in the first line.

The format for the line containing IVec is Fortran I5. The β orbital replacement section for UHF calculations differs only in that it omits the initial format specification line. See the examples section for sample replacement orbital input.

Input

Read a line from the input file containing the name of a checkpoint file. Instead of a filename, you can specify one of the following keywords:

• generate says to generate a guess as usual.
• read and chk say to retrieve data from the job’s checkpoint file. They are thus equivalent to Guess=Read, and use the checkpoint file specified to %Chk or %OldChk as usual.
• none says to skip generating a guess at all.
• dummy generates a dummy guess that does not require any work.

See the examples section for information about using this option with ONIOM.

Restart

Start a new SCF calculation using the restart data saved on the checkpoint file from a job with a different geometry and/or basis set. (The option name is a bit of a misnomer.)

Save

Save the generated initial guess back into the checkpoint file at the conclusion of a Guess=Only run. This option is useful for saving localized orbitals.

Print

Print the initial guess.

Symmetry-Related Options

LowSymm

Requests that irreducible representations of the molecular point group be combined in the symmetry information used in the N³ steps in the SCF to allow lowered symmetry of the wavefunction. This enables the orbitals (and possibly but not necessarily the total wavefunction) to have lower symmetry than the full molecular point group. This option is available only for GVB calculations, where it is often necessary for calculations on symmetric systems (see the discussion of the GVB keyword below for an example using this option).

The option expects a single line of input (in the format 16I2) giving the numbers of the irreducible representations to combine, with the new groups separated by 0; the list itself must be terminated by a 9. The numbers correspond to the order in which the representations are listed by Link 301 in the output file (see the examples subsection below).

Since this input section is always exactly one line long, it is not terminated by a blank line. Note that irreducible representations are combined before orbital localization is done and that localized orbitals retain whatever symmetry is kept. Guess=NoSymm removes all orbital symmetry constraints without reading any input.

NoSymm

Requests that all orbital symmetry constraints be lifted. Synonymous with SCF=NoSymm and Symm=NoSCF.

ForceAbelianSymmetry

Force the initial guess orbitals to transform according to irreducible representations of the Abelian point group. NoForceAbelianSymmetry is the default.

Valid Option Combinations

Only reasonable combinations are valid. For example Guess=(Always,Alter) and Guess=(Read,Alter) work as expected (in the former case, alterations are read once and the same interchanges are applied at each geometry). Conversely, Guess=(Always,Read) is contradictory and will lead to unpredictable results. Refer to the input sections order table at the beginning of this chapter to determine the ordering of the input sections for combinations of options like Guess=(Cards,Alter).

Restrictions

Guess=Only may not be used with the MOPAC-based semi-empirical methods: INDO, CNDO, MNDO, and MINDO3.

Related Keywords

Geom, Pop