Gaussian 03 Online Manual
This keyword controls the initial guess for the Hartree-Fock wavefunction. Guess is not meaningful without an option. By default, a Harris guess is used (see below).
For UHF calculations, two such orbital alteration sections are required, the first specifying transpositions ofα 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.
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.
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.
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.
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 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).
Guess=Only may not be used with semi-empirical methods.
Transposing 2 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, α and β orbitals are given separately. From the orbital symmetries, the electron configuration in the initial guess is 2a12a12b22a11b1, 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 β electron must be moved from orbital 4 to orbital 5 (the electron configuration is then 2a12a12b22b11a1).
Guess=Alter may also 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 α section(empty in this case). 4 5 Transpose orbitals 4 and 5. End of the β alteration section.
Note that an extra blank line—line 12—is necessary to indicate an empty α alteration section. The final two lines then constitute the β 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.
Reordering Orbitals with Guess=Permute. This option is often is the easiest way to perform a complex modification of the initial guess, as in this example:
# CASSCF/6-31G(d,p) Opt Guess=Permute Pop=Reg Test CAS job 0 1 molecule specification 1-60 65 63 64 66 68 67 61-62 69 Specify new ordering.
Here we have rearranged orbitals 61-68. Listing the final orbital (69) is not really necessary, but it help to make the input easier to understand for humans.
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.