$FFCALC

$FFCALC group (relevant for RUNTYP=FFIELD)

This group permits the study of the influence of an applied electric field on the wavefunction. The most common finite field calculation applies a sequence of fields to extract the linear polarizability and first and second order hyperpolarizability. The method is general, and so works for all ab initio wavefunctions in GAMESS.

EFIELD
= applied electric field strength (default=0.001 a.u.)

IAXIS and JAXIS
specify the orientation of the applied field. 1,2,3 mean x,y,z respectively. The default is IAXIS=3 and JAXIS=0.

If IAXIS=i and JAXIS=0, the program computes alpha(ii), beta(iii), and gamma(iiii) from the energy changes, and a few more components from the dipole changes. Five wavefunction evaluations are performed.

If IAXIS=i and JAXIS=j, the program computes the cross terms beta(ijj), beta(iij), and gamma(iijj) from the energy changes, and a few more components from the dipole changes. This requires nine evaluations of the wavefunction.

AOFF
= a flag to permit evaluation of alpha(ij) when the dipole moment is not available. This is necessary only for MP2, and means the off-axial calculation will do 13, not 9 energy evaluations. Default=.FALSE.

SYM
= a flag to specify when the fields to be applied along the IAXIS and/or JAXIS (or according to EONE below) do not break the molecular symmetry. Since most fields do break symmetry, the default is .FALSE.

ONEFLD
= a flag to specify a single applied field calculation will be performed. Only the energy and dipole moment under this field are computed. If this option is selected, only SYM and EONE input is heeded. The default is .FALSE.

EONE
= an array of the three x,y,z components of the single applied field.

There are notes on RUNTYP=FFIELD on the next page.

Finite field calculations require large basis sets, and extraordinary accuracy in the wavefunction. To converge the SCF to many digits is sometimes problematic, but we suggest you use the input to increase integral accuracy and wavefunction convergence, for example

             $CONTRL ICUT=20 ITOL=30 INTTYP=HONDO $END
             $SCF    NCONV=10 FDIFF=.FALSE. $END
 

In many cases, the applied fields will destroy the molecular symmetry. This means the integrals are calculated once with point group symmetry to do the initial field free wavefunction evaluation, and then again with point group symmetry turned off. If the fields applied do not destroy symmetry, you can avoid this second calculation of the integrals by SYM=.TRUE. This option also permits use of symmetry during the applied field wavefunction evaluations.

Examples of fields that do not break symmetry are a Z-axis field for an axial point group which is not centrosymmetric (i.e. C2v). However, a second field in the X or Y direction does break the C2v symmetry. Application of a Z-axis field for benzene breaks D6h symmetry. However, you could enter the group as C6v in $DATA while using D6h coordinates, and regain the prospect of using SYM=.TRUE. If you wanted to go on to apply a second field for benzene in the X direction, you might want to enter Cs in $DATA, which will necessitate the input of two more carbon and hydrogen atom, but recovers use of SYM=.TRUE.

Reference: H.A.Kurtz, J.J.P.Stewart, K.M.Dieter J.Comput.Chem. 11, 82-87 (1990).

For analytic computation of static and also frequency dependent NLO proerties, for closed shell cases, see the $TDHF group.


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