Guess Keyword
Last Update:
12/31/2000
DESCRIPTION
This keyword controls the initial guess for the Hartree-Fock
wavefunction. Guess is not meaningful without an option. By default, an
INDO guess is used for first-row systems, CNDO for second-row, and Huckel for
third-row and beyond.
OPTIONS
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
orbitals, and the second specifying transpositions of
orbitals. Both sections are always required. Thus, even if only transpositions
are needed, the
section is expected even though it is empty (and vice-versa). The second blank
line to indicate an empty
section must be included.
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
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.
Mix Requests that the HOMO and LUMO be mixed so as to
destroy
  and spatial symmetries. This is useful in producing UHF
wavefunctions for singlet states.
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.
Local Requests that orbitals be localized using the
Boys method [202]. 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.
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.
Core Requests that the core Hamiltonian be diagonalized
to form the initial guess. Guess=Core is most commonly used for atomic
calculations.
Huckel Requests that a pseudo-extended Huckel guess be
generated.
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
and
replacement orbital input sections.
The replacement orbitals input section (the
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 to 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
orbital replacement section for UHF calculations differs only in that it omits
the initial format specification line. See the examples subsection for sample
replacement orbital input.
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
semi-empirical methods.
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.
Save Saves 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.
These options may be combined in any reasonable combination. Thus
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 input sections
sequence 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 semi-empirical methods.
RELATED KEYWORDS
Geom, Pop
EXAMPLES
Transposing Orbitals with Guess=Alter. This example finds
the UHF/STO-3G structure of the 2A1 excited state of the
amino radical. First, a Guess=Only calculation is run to determine
whether any alter instructions are needed to obtain the desired electronic
state. The HF/STO-3G theoretical model is used by default:
# Guess=Only Test
Amino radical test of initial guess
0 2
n
h 1 nh
h 1 nh 2 hnh
nh 1.03
hnh 120.0
Here is the orbital symmetry summary output from the job, which
comes immediately before the population analysis in the output:
Initial guess orbital symmetries.
Alpha Orbitals:
Occupied (A1) (A1) (B2) (B1) (A1)
Virtual (A1) (B2)
Beta Orbitals:
Occupied (A1) (A1) (B2) (A1)
Virtual (B1) (A1) (B2)
<S**2> of initial guess= .7544
Since a doublet state is involved,
and
orbitals are given separately. From the orbital symmetries, the electron
configuration in the initial guess is
a12a12b22a12b1
yielding a 2B1 wavefunction. This is indeed the ground
state of NH2. The expectation value of S2 for the
unrestricted initial guess is printed. In this case, it is close to the pure
doublet value of 0.75.
Note that the orbital energies printed in a Guess=Only job
are simply -1.0 for the occupied orbitals and 0.0 for the virtual orbitals,
since no SCF has been performed. If the actual orbital energies are desired, a
full semi-empirical energy calculation can be performed specifying the desired
method (e.g. INDO).
Returning to our consideration of the amino radical, since we want
to model the 2A1 excited state, we will need to alter
this initial orbital configuration: a
electron must be moved from orbital 4 to orbital 5 (the electron configuration
is then
a12a12b22b12a1).
Guess=Alter may be used to accomplish this. Here is the input for the
geometry optimization
# UHF/6-31G(d) Opt Guess=Alter Pop=Reg Test
Amino radical: HF/6-31G(d) structure of 2-A1 state
0 2
n
h 1 nh
h 1 nh 2 hnh
Variables:
nh 1.03
hnh 120.0
Blank line ends the molecule specification section.
Blank line ends the alpha section (empty in this case).
4 5 Transpose orbitals 4 and 5.
End of the beta alteration section.
Note that an extra blank line--line 12--is necessary to indicate
an empty
alteration section. The final two lines then constitute the
alteration section.
The initial guess program prints a list of orbitals that were
interchanged as a result of the Alter option:
Projected INDO Guess.
NO ALPHA ORBITALS SWITCHED.
PAIRS OF BETA ORBITALS SWITCHED:
4 5
The eigenvalue of S2 is printed for the UHF
wavefunction. The value which results if contamination of the wavefunction from
the next possible spin multiplicity (quartets for doublets, quintets for
triplets, etc.) is removed is also printed:
Annihilation of the first spin contaminant:
S**2 before annihilation .7534, after .7500
Although this calculation does in fact converge correctly to
2A1 state, it sometimes happens that the order of orbital
symmetries switches during the course of the SCF iterations. If the orbital
symmetries of the final wavefunction are different from those in the initial
guess (whether or not you are using Guess=Alter), we recommend using the
direct minimization routine, specified with the SCF=QC or SCF=DM
keywords, which usually holds symmetry from one iteration to the next.
Reading in Orbitals with Guess=Cards. Some or all of the
orbitals may be replaced after the initial guess is generated using
Guess=Cards. Here is some sample input for this option, which replaces
orbitals 1 and 4 (note that the format for the third and following lines is
specified in line 1):
(3E20.8)
1
0.5809834509E+00 0.4612416518E+00 -0.6437319952E-04
0.1724432549E-02 0.1282235396E-14 0.5417658499E-13
0.1639966912E-02 -0.9146282229E-15 -0.6407549694E-13
-0.4538843604E-03 0.6038992958E-04 -0.1131035485E-03
0.6038992969E-04 -0.1131035471E-03
4
0.7700779642E-13 0.1240395916E-12 -0.3110890228E-12
-0.4479190461E-12 -0.1478805861E-13 0.5807753928E+00
0.6441113412E-12 -0.3119296374E-14 0.1554735923E+00
-0.1190754528E-11 0.2567325943E+00 0.1459733219E+00
-0.2567325943E+00 -0.1459733219E+00
0
An orbital number of zero ends the replacement orbital input.
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