Doubly excited states have nowadays become important in technological applications, e.g., in increasing the efficiency of solar cells and therefore, their description using ab initio methods is a great theoretical challenge as double excitations cannot be described by linear response theories based on a single Slater determinant. In the present work we extend our recently developed Hartree-Fock (HF) approximation for calculating singly excited states[M. Tassi, I. Theophilou, and S. Thanos, Int. J. Quantum Chem.113, 690 (Year: 2013)10.1002/qua.24049] in order to allow for the calculation of doubly excited states. We describe the double excitation as two holes in the subspace spanned from the occupied HF orbitals and two particles in the subspace of virtual HF orbitals. A subsequent minimization of the energy results to the determination of the spin orbitals of both the holes and the particles in the occupied and virtual subspaces, respectively. We test our method, for various atoms, H2 and polyene molecules which are known to have excitations presenting a significant double excitation character. Importantly, our approach is computationally inexpensive.
Received 25 January 2013Accepted 08 March 2013Published online 27 March 2013
The authors would like to thank Professor A. K. Theophilou and Dr. N. Helbig for discussions on the manuscript and Dr. N. Lathiotakis for comments on the manuscript and for his help with the computational part of this work.
Article outline: I. INTRODUCTION II. METHODOLOGY AND NUMERICAL IMPLEMENTATION A. Imposed restrictions to the excited state determinant for the excitation of two electrons with different spin B. Minimization equations C. Distinction of subspaces in which Fock matrix diagonalization is performed D. Excitation of two electrons with same spin III. RESULTS AND DISCUSSION IV. CONCLUSIONS
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