Computational studies CASSCF calculations

In a combined experimental/computational study, the vibrational spectra of the N9H and N7H tautomers of the parent purine have been investigated [99SA(A) 2329]. Solvent effects were estimated by SCRF calculations. Vertical transitions, transition dipole moments, and permanent dipole moments of several low-lying valence states of 2-aminopurine 146 were computed using the CIS and CASSCF methods [98JPC(A)526, 00JPC(A)1930]. While the first excited state of adenine is characterized by an n n transition, it is the transition for 146. The... 

The methodology that uses the dielectric model is instead the simpler and in principle the more suitable for the study of chemical reactions involving large molecular systems. In 1998, Amovilli et al [13] developed a computer code in which the solvent reaction field, including all the basic solute-solvent interactions, has been considered for Complete Active Space Self Consistent Field (CASSCF) calculations. 

The CASSCF wavefunction is then transcribed in a VB formalism using a restricted set of stmctnres which are snfficient to describe correctiy the chemical bonds of reactants and prodncts involved in the reaction. This VB analysis, from a computational point of view, is also an important test for the CASSCF calculation itself because, working on the active space, it can tell us if the choice of the active orbitals is appropriate to study the reaction nnder investigation the full correlation energy, in fact, is very large, if compared with the energy involved in a chanical process, and a wrong active space can lead to a completely unbalanced PES. 

All calculations were carried out with the software MOLCAS-6.0 [16]. Scalar relativistic effects were included using a DKH Hamiltonian [14,15]. Specially designed basis sets of the atomic natural orbital type were used. These basis sets have been optimized with the scalar DKH Hamiltonian. They were generated using the CASSCF/CASPT2 method. The semi-core electrons (ns, np, n — 3,4, 5) were included in the correlation treatment. More details can be found in Refs. [17-19]. The size of the basis sets is presented in Table 1. All atoms have been computed with basis sets including up to g-type function. For the first row TMs we also studied the effect of adding two h-type functions. 

These results and also the results for the excitation energies will of course be modified by dynamical correlation effects, which will be accounted for by means of MR-CI calculations based on the CASSCF orbitals for each state. The transition moments will be computed using the CASSI method, where the two A are allowed to interact. The final results can be expected to be accurate to within 0.1 eV for the excitation energies and 10% for the transition moments. The study of the CCCN system is not yet finished. Hopefully some result can be presented in the lectures. 

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