CASSCF keyword

Molecular frequencies depend on the second derivative of the energy with respect to the nuclear positions. Analytic second derivatives are available for the Hartree-Fock (HF keyword). Density Functional Theory (primarily the B3LYP keyword in this book), second-order Moller-Plesset (MP2 keyword) and CASSCF (CASSCF keyword) theoretical procedures. Numeric second derivatives—which are much more time consuming—are available for other methods. 

A CASSCF calculation is requested in Gaussian with the CASSCF keyword, which requires two integer arguments the number of electrons and the number of orbitals in the active space. The active space is defined assuming that the electrons come from as many of the highest occupied molecular orbitals as are needed to obtain the specified number of electrons any remaining required orbitals are taken from the lowest virtual orbitals. 

C=C stretch 80 C-13 chemical shifts 22, 53 C60 31,32 C60O isomers 54 carbon dioxide 120, 182 carbon monoxide 175,191 carbonyl series 84 carbonyl stretch 84, 220 in solution 244 Carmichael 136 Carpenter 152, 196 Cartesian coordinates 52, 286, 287 CASSCF keyword 228 CASSCF method 228,229,230,231, 232,233, 234,235 state-averaged 233... 

For example, in a 4-electron, 6-orbital CAS—specified as CASSCF 4,6)—performed on a singlet system, the active space would consist of the two highest occupied molecular orbitals (where the four electrons reside) and the four lowest virtual orbitals. Similarly, for a 6-electron, 5-orbital CAS on a triplet system, the active space would consist of the four highest occupied MO s— two of which are doubly-occupied and two are singly-occupied—and the LUMO (the keyword is CASSCF(6,5)). 

Step 3 is a geometry optimization. Appropriate keywords might be CASSCF (2,2)/6-31G, specifying a CASSCF(2,2) procedure (a limited Cl optimization) using the 6-31G basis, which will normally be the smallest chosen. Other keywords might dictate the information to be taken from step 2 and how to calculate the initial Hessian (e.g., use a semiempirical calculation) for the optimization. Figure 8.11 compares our CASSCF(2,2)/6-31G C2h relative minimum (no imaginary frequencies - see below) with the C2h CASSCF(4,4)/6-31G minimum of Doubleday [61]. 

Keywords Ehrenfest method CASSCF Coupled electron-nuclear dynamics Charge migration Charge transfer... 

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