$EFIELD group (not required) This group permits the study of the influence of an external electric field on the molecule. The method is general, and so works for all wavefunctions, and for both energies and nuclear gradients. EVEC = an array of the three x,y,z components of the applied electric field, in a.u., where 1 Hartree/e*bohr = 5.1422082(15)D+11 V/m A typical size for the EVEC components is therefore about 0.001 a.u. SYM = a flag to specify when the field breaks the the molecular symmetry. Since most fields break symmetry, the default is .FALSE. ========================================================== Restrictions: analytic hessians are not available, but numerical hessians are. Because an external field causes a molecule with a dipole to experience a torque, geometry optimizations must be done in Cartesian coordinates only. Internal coordinates eliminate the rotational degrees of freedom, which are no longer free. A nuclear hessian calculation will have two rotational modes with non-zero "frequency", caused by the torque. A gas phase molecule will rotate so that the dipole moment is anti-parallel to the applied field. To carry out this rotation during geometry optimization will take many steps, and you can help save much time by inputting a field opposite the molecular dipole. There is also a stationary point at higher energy with the dipole parallel to the field, which will have two imaginary frequencies in the hessian. These will appear as the first two modes in a hessian run, but will not have the i for imaginary included on the printout since they are rotational modes. sign conventions: Dipole vectors are considered to point from the negative end of the molecule to the positive end. Thus HCl at the MP2/aug-cc-pVDZ level's geometry of R=1.2831714 has a positive dipole, if we place Cl at the origin and H along the positive z-axis. The sign convention on applied fields is such that a +1 charge particle feels a force in the positive direction under a positive field, namely, as if there was a negative plate at large +Z and a positive plate at large -Z. Hence positive fields enhance HCl's dipole: EVEC(z) E(MP2) mu(MP2) -0.001 -460.2567905970 1.112875 -0.0001 -460.2571917846 1.153172 0.0 -460.2572372416 1.157646 +0.0001 -460.2572828745 1.162119 +0.001 -460.2577014871 1.202350 and the higher energy for each negative EVEC means HCl would prefer to turn around in the field. Thus, one use for this group is calculation of the electric dipole by finite difference, for wavefunctions that cannot yield molecular properties due to not having a relaxed density matrix. Perform two RUNTYP=ENERGY jobs per component, with fields 0.001 and -0.001 a.u. The central difference formula for each component of the dipole is mu = 2.541766*(E(+0.001)-E(-0.001)/0.002, in Debye. The differentiation using data from HCl gives 1.157635. For an application to molecular ionization in intense fields generated by lasers, see H.Kono, S.Koseki, M.Shiota, Y.Fujimura J.Phys.Chem.A 105, 5627-5636(2001) ==========================================================

generated on 7/7/2017