Molecular dynamics simulations of Ca2+ in water:: Comparison of a classical simulation including three-body corrections and Born-Oppenheimer ab initio and density functional theory quantum mechanical/molecular mechanics simulations

A classical molecular dynamics simulation including three-body corrections was compared with combined ab initio quantum mechanics/molecular mechanics molecular dynamics simulations (QM/MM-MD), which were carried out at Hartree-Fock (HF) and density functional theory (DFT) level for Ca2+ in water. In the QM approach the region of primary interest-the first hydration sphere of the calcium ion-was treated by Born-Oppenheimer quantum mechanics, while the rest of the system was described by classical pair potentials. Coordination numbers of 7.1, 7.6, and 8.1 were found in the classical, the HF, and the DFT simulation, respectively, using the same double-zeta basis set in both QM methods. The CPU time for one DFT step was about 50% above the time for a HF step, but due to a smaller number of steps needed for equilibration in the DFT case, there was no significant difference in the overall simulation time. (C) 2001 American Institute of Physics.