Quasidiabatic states from a b i n i t i o calculations by block diagonalization of the electronic Hamiltonian: Use of frozen orbitals

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Abstract
The construction of quasidiabatic states is of theoretical and practical relevance in treating systems that exhibit strong nonadiabatic interactions. In the present work the implementation of the recently proposed method of block diagonalization of the electronic Hamiltonian in CI‐type calculations, both theoretically and in practical calculations is investigated in detail. It is shown that the use of shifted or frozen molecular orbitals in the calculation of the adiabatic potential energy surfaces provides the most appropriate N‐electron wave functions for applying the block diagonalization procedure. Freezing means to conserve the composition of the molecular orbitals at some chosen geometry, i.e., to have their coefficients unchanged with respect to the atomic orbitals with varying nuclear geometry. The use of the frozen orbitals and block diagonalization of the resulting CI matrix yields a method that treats orbital coupling and configurational interaction efficiently at the same time, leading to quasidiabatic states. Due to the mathematical simplicity of the method and to the fact, that no derivative couplings are needed during the calculations, the additional computational cost is negligible. Application to the ethylene dication illustrates the successful performance of the proposed method.