$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.
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.
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.