PURPOSE: To eliminate the need for a step wherein a Fock matrix is diagonilized by providing a step wherein the analytic output of a molecular system is generated when the convergence of the energy of the molecular system within specific limits is determined.

CONSTITUTION: The Fock matrix 7 is generated on the basis of a 1-electron matrix 2,2-electron integration 3, and a density matrix 6. Then an erasing process is performed by using an initial localized molecular orbital 5. This erasing process is basically to diagnonilize a 2×2 matrix. Here, Ψi is a coefficient regarding a molecular orbital (i) and Ψj* is a coefficient regarding an antibonding molecular orbital (j). The 2×2 diagonalizing is performed so as to reduce all possible interactions between bonding orbitals and antibonding orbitals. Consequently, localized molecular orbitals are formed and used to the density matrix 6 and Fock matrix. Then erasure for repeatedly erasing all the antibonding orbitals and all interactions of bonding is carried out. The erasing process includes the simple diagnolizing of the 2×2 matrix.