The water dimer interaction energy and its convergence to the basis set limit was investigated, with electron correlation effects treated at the level of second order Mo "ller-Plesset perturbation theory ͑MP2͒. ANO-type and large uncontracted basis sets were used, spreading over a wide range in size; the biggest set included 1046 functions with angular momentum up to (lϭ7). Core correlation effects were treated accurately by augmenting the original valence basis with extended sets of core polarization functions. The MP2 dimer interaction energy at the basis set limit was determined to Ϫ4.94Ϯ0.02 kcal/mol, with a contribution due to core correlation of Ϫ0.04 kcal/mol. Furthermore, based on some elementary considerations from intermolecular perturbation theory, a simple procedure was devised, which brings the counterpoise corrected interaction energies of moderate basis set calculations closer to the basis set limit. The interaction energies so obtained turned out be surprisingly stable with respect to extensions of the basis set. © 1997 American Institute of Physics. ͓S0021-9606͑97͒00835-0͔

The solvations of positively and negatively charged model ions in water droplets have been studied using Monte Carlo simulations performed with a polarizable intermolecular potential function model. Special focus has been placed on the position of the ion in the water droplet. It was found that the sign of the ionic charge is of minor importance but an increased ionic charge localizes the ion to the central regions of the droplet, whereas a large polarizability and a large ionic radius favor locations close to the surface of the water droplet.

The nonempirical force field model (NEMO) has been augmented with a new exchange repulsion model based on a different choice of expansion center. In the new repulsion model, the size of the atoms are estimated from the trace of the local second-order electron density tensor divided with the local valence charge. A set of general atom type dependent parameters are fitted from SCF calculations on different complexes. An exponential form is used to describe the interaction. The new expansion center is chosen as the point where the local electronic dipole is zero, i.e. the center of electronic charge. The model has been tested on a set of intermolecular complexes between the molecules water, ammonia, methylamine, formamide, and urea.

An intramolecular NEMO potential is presented for the N-formylglycinamide molecule together with an intermolecular potential for the N-formylglycinamide-water system. The intramolecular N-formylglycinamide potential can be used as a building block for the backbone of polypeptides and proteins. Two intramolecular minima have been obtained. One, denoted as C5, is stabilized by a hydrogen bonded five member ring, and the other, denoted as C7, corresponds to a seven membered ring. The interaction between one water molecule and the N-formylglycinamide system is also studied and compared with Hartree-Fock SCF calculations and with the results obtained for some of the more commonly used force fields. The agreement between the NEMO and SCF energies for the complexes is in general superior to that of the other force fields. In the C7 region the surfaces obtained from the intramolecular part of the commonly used force fields are too flat compared to the NEMO potential and the ab initio calculations. We further analyze the possibility of using a charge distribution obtained from one conformation to describe the charge distribution of other conformations. We have found that the use of polarizabilities and generic dipoles can model most of the changes in charge density due to the different geometry of the new conformations, but that one can expect additional errors in the interaction energies that are of the order of 1 kcal/mol.

A test of the quality of the electrostatic properties and polarizabilities used in the nonempirical molecular orbital (NEMO) potential is carried out for formamide by calculating the molecular dipole moment and polarizability at the second-order Mùller±Plesset (MP2) level of theory. The molecular dipole moment is 11% lower at the MP2 level than at the Hartree±Fock (HF) level, whereas the isotropic part of the polarizability is increased by 36% by adding electron correlation and using a considerably larger basis set. The atomic charges, dipole moments and polarizabilities obtained at the HF level are rescaled to get the correct molecular properties at the MP2 level. The potential minimum for the cyclic dimer of formamide is À17:50 kcal/mol with the MP2-scaled properties and is signi®cantly lower than other potentials give. Two intermolecular potentials are constructed and used in subsequent molecular dynamics simulations: one with the regular NEMO potential and the other with the rescaled MP2 properties. A damping of the electrostatic ®eld at short intermolecular distances is included in the present NEMO model. The average energies for liquid formamide are lower for the MP2-scaled model and are in good agreement with experimental results. The lowering of the simulation energy for the MP2-scaled potential indicates the strong dispersive interactions in liquid formamide.