Van der waals bound states and intermolecular bend-stretch coupling in NeC2H4 and Ar-tetrazine

Solvent configuration-interaction calculations of intermolecular states in molecule-(atom)N clusters: application to Br2-4HeN

We describe variational calculations of J=0 intermolecular states in Br(2)-(4)He(N) clusters. The method employed is analogous to configuration-interaction calculations in electronic-structure work and relies on the ability to express the intermolecular Hamiltonian H(v) as a sum of one- and two-body terms. A basis set is built up from solutions to the Schrödinger equation in which only the one-body terms of H(v) are included. These configurations are products of N=1 eigenstates. The matrix of H(v) in a symmetry-adapted configuration basis is then computed, the two-body terms of H(v) serving to couple different configurations. This computation involves integrals of dimension five or less. Filter diagonalization is then used to obtain energies and eigenfunctions within a selected energy range. Results on clusters having N=2-5 are reported.

Potential-energy surface, van der Waals energy spectrum, and vibronic transitions in s-tetrazine-argon complex

The van der Waals vibrational states and the structure of the vibronic spectrum of s-tetrazine-argon complex have been studied by the ab initio methods. The potential-energy surface of the ground S(0) electronic state of the complex has been constructed by fitting the analytical many-body expansion to a large set of the interaction energy values computed using the second-order Møller-Plesset perturbation theory combined with the standard aug-cc-pVDZ basis set. The equilibrium structure of the complex found is that with argon located above the tetrazine ring at a distance of 3.394 A. The calculated dissociation energy of 354 cm(-1) is compatible with the experiment. The van der Waals energy spectrum calculated from the potential-energy surface is explained analyzing a correlation with a simpler energy spectrum of benzene-argon. A new assignment of the S(0)-S(1) vibronic spectrum is proposed on the basis of the rigorous selection rules, vibrational energy levels in S(0) and S(1) states and vibronic transition intensities calculated from the electronic transition dipole moment surfaces.

Fully dimensional ab initio description of the structure and energetics of azabenzene-argon complexes

The structure and energetics of van der Waals complexes of argon with azabenzenes: pyridine, pyrazine, pyrimidine, pyridazine, s-triazine, and s-tetrazine are studied using the second-order Moller-Plesset perturbation theory combined with well-balanced basis sets. The full optimization of the cluster structures and computation of the inter- and intramolecular vibrational frequencies is performed by eliminating the basis set superposition error. The argon equilibrium coordinates are calculated with the accuracy comparable to that reached by standard methods of the structure determination from the spectral data. A simple rule to predict the position of argon with respect to the geometric center of the azabenzene ring is found. The calculated harmonic frequencies of the intermolecular vibrational modes are scaled by the factor of 0.85 to eliminate systematic errors coming from the neglect of anharmonic effects. The scaled frequencies agree with the experimental ones to about 1 cm(-1), except for pyrimidine-argon and tetrazine-argon for which empirical fundamental frequency estimates are problematic. A simple relation connecting the intermolecular bending frequencies and the monomer quadrupole tensor is found. The perturbation of the monomer properties caused by complexation is analyzed. The modification of the monomer structure by the interaction with argon and its influence on the binding energy appears to be negligible in all complexes studied. However, this interaction affects appreciably the intramolecular modes and causes their frequency shifts. As a consequence, the dissociation energy of the complexes increases by about 5 cm(-1).