Quantum mechanical/quantum mechanical methods. I. A divide and conquer strategy for solving the Schrodinger equation for large molecular systems using a composite density functional-semiempirical Hamiltonian

Herein we describe a new combined quantum mechanical/quantum mechanical (QM /QM) method for solving the Schrodinger equation for large molecular system s. The new method uses the divide and conquer (D&C) strategy to partition a large molecular system into subsystems and a composite density functional theory (DFT)-semiempirical (SEM) Hamiltonian to describe the molecular inte ractions. The DFT and SEM subsystems are coupled through the chemical poten tial and are equilibrated by exchanging electronic charge. Calculations per formed with the DFT, SEM, and composite (DFT/SEM) methods on diatomic, tria tomic, and polyatomic molecules show that as one moves away from the QM/QM boundary region the Mulliken charges converge to the values that would be o btained using the "pure" Hamiltonian. In other words, we find that the qual ity of each SEM and DFT wave function is largely conserved, which strongly suggests that this type of approach could be applied to study chemical reac tivity much in the same way combined quantum mechanical/molecular mechanica l (QM/MM) methods are presently utilized. (C) 2000 American Institute of Ph ysics. [S0021-9606(00)30438-X].