$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 the first and second order hyperpolarizabilities (static alpha, beta, and gamma tensors). The method is general, because it relies on finite differencing of the energy values, and so works for all ab initio wavefunctions. If the dipole moments are available (true for SCF or CI functions, and see MPPROP in $MP2), the same tensors are formed by differencing the dipoles, which is more accurate. Some idea of the error in the numerical differentiations can be gleaned by comparing energy based and dipole based quantities. For analytic computation of static polarizabilities alpha, beta, and gamma (as well as frequency dependent NLO properties), for closed shell cases, see $TDHF and $TDHFX. For analytic computation of the static polarizability alpha, see POLAR in $CPHF. The standard computation obtains the polarizabilities, by double numerical differentiation. See ONEFLD to apply a single electric field, but for a more general approach to applied static fields, see $EFIELD. OFFDIA = .TRUE. computes the entire polarizability tensors, which requires a total of 49 wavefunction evaluations (some of gamma is not formed). = .FALSE. forms only diagonal components of the polarizabilities, using 19 wavefunctions. The default is .TRUE. ESTEP = step size for the applied electric field strength, 0.01 to 0.001 is reasonable. (default=0.001 a.u.) The next parameters pertain to applying a field in only one direction: ONEFLD = flag to apply one field (default=.FALSE.) SYM = a flag to specify when the field to be applied does not break the molecular symmetry. Since most fields do break the nuclear point group symmetry, the default is .FALSE. EFIELD = an array of the three x,y,z components of the single applied field. * * * LOCHYP = a flag to perform a localized orbital analysis of the alpha, beta, and gamma polarizabilities. See $LOCAL for similar analyses of the energy, multipole moments, or alpha polarizability. References for this keyword are given below. 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 $END $SCF CONV=1d-7 FDIFF=.FALSE. $END 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 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. References: J.E.Gready, G.B.Bacskay, N.S.Hush Chem.Phys. 22, 141-150(1977) H.A.Kurtz, J.J.P.Stewart, K.M.Dieter J.Comput.Chem. 11, 82-87(1990). polarizability analysis: S.Suehara, P.Thomas, A.P.Mirgorodsky, T.Merle-Mejean, J.C.Champarnaud-Mesjard, T.Aizawa, S.Hishita, S.Todoroki, T.Konishi, S.Inoue Phys.Rev.B 70, 205121/1-7(2004) S.Suehara, T.Konishi, S.Inoue Phys.Rev.B 73, 092203/1-4(2006) ========================================================== ==========================================================

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