Simulation MCSCF

The integral equation method is free of the disadvantages of the continuum model and simulation techniques mentioned in the foregoing, and it gives a microscopic picture of the solvent effect within a reasonable computational time. Since details of the RISM-SCF/ MCSCF method are discussed in the following section we here briefly sketch the reference interaction site model (RISM) theory. 

The MPE/MCSCF approach has been employed to study the interplay of solvent and conformation effects on the spin-spin coupling constants in methanol and methylamine [72], The simulated solvent effects are noticeable for the one-bond coupling constants and for some of the geminal coupling constants but negligible for 3/ee. The dielectric continuum effects have been found to depend considerably on the molecular conformation in the case of and 27ece. It is worth noting here that the MCSCF results have confirmed the conclusions drawn in ref. [80] from semi-empirical continuum model calculations. 

RISM theory can be regarded as an alternative to the molecular simulation method, while the RISM-SCF/MCSCF method can be considered as an alternative to the QM/MM method. It is important to note that the method is derived from a natural extension of the RISM theory as well as the ab initio theory. In this section, after introducing the method, we will show some representative examples. 

It now seems hard to believe that, 20 years ago, (6/6)CASSCF/3-21G calculations were beyond the capabilities of the computers available to most computational chemists. Nevertheless, in the first ab initio study of the Cope rearrangement, published in 1984, (6/6)CASSCF/3-21G calculations could not actually be performed they could only be simulated [14]. Smaller MCSCF calculations were employed to obtain a partially optimized set of orbitals. Then those orbitals were used to carry out full Cl calculations for the 52 configurations that comprise the complete (6/6) active space at the C h stationary point for the chair Cope rearrangement of 1,5-hexadiene. 

The 3D-RISM-MCSCF approach has been applied to carbon monoxide (CO) solute in ambient water [33]. Since it is known that the Hartree-Fock method predicts the electronic structure of CO in wrong character [167], the CASSCF method (2 core, 8 active orbitals, 10 electrons) in the basis sets of double zeta plus polarization (9s5pld/4s2pld) augmented with diffuse functions (s- and p-orbitals) was used. Water was described by the SPC/F model [127] and the site-centered local pseudopotential elaborated by Price and Halley for CP simulation [40]. The 3D-RISM/KH integral equations for the water distributions specified on a grid of 64 points in a cubic supercell of size 20 A were solved at each step of the SCF loop by using the method of modified direct inversion in the iterative subspace (MDIIS) [27, 29] (see Appendix). 

Several applications will be reviewed and discussed in the following sections. Specific information about methods used will be given. Details can be found in the proper references. The simulations carried out by the present authors have been performed with the Newton-X program (Barbatti et al. 2007a, b) for mixed quantum-classical dynamics simulations, interfaced with the Columbus program (Lischka et al. 1981, 2001, 2006) for MRCl and MCSCF electronic structure calculations. 

---