Configuration functions multiconfigurational second-order

R. Cammi and J. Tomasi, Nonequilibrium solvation theory for the polarizable continuum model - a new formulation at the SCF level with application to the case of the frequency-dependent linear electric-response function, Int. J. Quantum Chem., (1995) 465-74 B. Mennucci, R. Cammi and J. Tomasi, Excited states and solvatochromic shifts within a nonequilibrium solvation approach A new formulation of the integral equation formalism method at the self-consistent field, configuration interaction, and multiconfiguration self-consistent field level, J. Chem. Phys., 109 (1998) 2798-807 R. Cammi, L. Frediani, B. Mennucci, J. Tomasi, K. Ruud and K. V. Mikkelsen, A second-order, quadratically... 

The above definition of electron correlation makes a generalization to the multiconfigurational case possible. To explain this, we return to the hydrogen molecule. We can easily compute the second order density from the two-configurational wave function (26) it is just the square of the wave functions. We obtain after spin integration ... 

The second problem centers about the use of an approximate ground-state wave function that eminates from a multiconfigurational zeroth-order approximation. The N2 calculations in Section III.C suggest that the restriction to a single configuration zeroth-order ground state imposes a fundamental limitation on the quality of the calculated EOM ionization potentials for that system. 

The most popular way of including dynamic correlation upon a CASSCF reference wave function [54-57] is the second-order perturbation theory (CASPT2) developed by Roos and coworkers [58]. However, in contrast to the single-configurational case, where the definition of the zeroth-order Hamiltonian is universal and taken as the sum of the one-electron Fock operators, the generalization of the zeroth-order Hamiltonian to the multiconfigurational case is not straightforward [59, 60]. A different, theoretically more justified approach is to... 

AIMD = ab initio molecular dynamics B-LYP = Becke-Lee-Yang-Parr CCSD = coupled cluster single double excitations CVC = core-valence correlation ECP = effective core potential DF = density functional GDA = gradient corrected density approximation MCLR = multiconfigurational linear response MP2 = M0ller-Plesset second-order (MRD)CI = multi-reference double-excitation configuration interaction RPA = random phase approximation TD-MCSCF = time-dependent multiconfigurational self-consistent field TD-SCF = time-dependent self-consistent field. 

Most of the theory in the present chapter is concerned with perturbational corrections to states that are dominated by a single electronic configuration, usually represented by a Hartree-Fock wave function. However, in Section 14.7, we consider multiconfigurational generalizations of Mpller-Plesset perturbation theory, in particular the second-order perturbation theory developed within the framework of CASSCF theory. 

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