Anchoring the water dimer potential energy surface with explicitly correlated computations and focal point analyses

Abstract

Ten stationary points on the water dimer potential energy surface have been characterized with the coupled-cluster technique which includes all single and double excitations as well as a perturbative approximation of triple excitations [CCSD(T)]. Using a triple-ζ basis set with two sets of polarization functions augmented with higher angular momentum and diffuse functions [TZ2P(f,d)+dif], the fully optimized geometries and harmonic vibrational frequencies of these ten stationary points were determined at the CCSD(T) theoretical level. In agreement with other ab initio investigations, only one of these ten stationary points is a true minimum. Of the other nine structures, three are transition structures, and the remaining are higher order saddle points. These high-level ab initio results indicate that the lowest lying transition state involved in hydrogen interchange is chiral, of C1 symmetry rather than Cs as suggested by recently developed 6D potential energy surfaces. The one- and n-particle limits of the electronic energies of these ten stationary points were probed by systematic variation of the atomic orbital basis sets and the treatment of electron correlation within the framework of the focal-point analysis of Allen and co-workers. The one-particle limit was approached via extrapolation of electronic energies computed with the augmented correlation consistent basis sets (aug-cc-pVXZ, X=D−6), and, independently, by estimating the basis set incompleteness effect with the explicitly-correlated second-order Møller-Plesset method (MP2-R12). Electron correlation was evaluated at levels as high as the Brueckner coupled cluster method with double excitations and perturbatively treated triple and quadruple excitations [BD(TQ)]. Core correlation and relativistic effects were also assessed. Consideration of the aforementioned electronic effects as well as basis set superposition error leads to an estimate of 21.0 kJ mol−1 for the electronic dissociation energy of (H2O)2.

Spectra of water dimer from a new ab initio potential with flexible monomers

“…29) or 12 cm −1 (Ref. 30). One should mention here that simple harmonic calculations predict IR frequency shifts reasonably well, in contrast to predictions of the intermonomer transition frequencies.…”

“…The symbol q X stands for the set of three internal coordinates of monomer X(= A, B), and q ref refers to a reference geometry, for which we take the vibrationally averaged structure. The quality of this flexible-monomer potential surface can be checked, among other things, by comparing the characteristics of the global minimum with the benchmark calculations of Tschumper et al 30 Table I shows (the dimer interatomic distances and angles are defined in Fig. 1) that intermonomer parameters are reproduced by SAPT-5s fIR with a few-percent errors: the binding energy D e with 5.2%, the 32 that deficiencies of SAPT-5s compared to CCpol-5s result mainly from the smaller basis set used to develop the former potential.…”

“…The effect of this symmetrization on the spectra is very small: most of the IR shifts change by less than 1 cm −1 , the largest change is 3 cm −1 for the O-H b [D] mode. The results for the intramolecular modes, listed in Table VII, can be compared with the best harmonic frequencies in the literature computed directly at CCSD(T) level in an augmented basis set of between doubleand triple-zeta quality by Tschumper et al 30 Note that in Ref. 30 the energies of the characteristic points are highly converged benchmark values, but the harmonic frequencies can probably be appreciably improved by using larger basis sets.…”

Generalization of the Gaussian electrostatic model: Extension to arbitrary angular momentum, distributed multipoles, and speedup with reciprocal space methods

“…To this end, for any tolerance δ we can define an effective radius R δ of the cluster with ∫r>R δ ρ ∼ tot (r)dr < δ. (27) Let T>2R δ , and for any site s∈S n and any α, t, u, and v, define the periodic extension of …”

Towards a force field based on density fitting

“…This contribution is generally misrepresented, diminishing the anisotropy of the interactions. 1 van Duijneveldt-van de Rijdt et al 9 recently tested several available molecular mechanics potentials on the difficult ten stationary points of the water dimer potential energy surface as described by Tschumper et al 10 They concluded that in order for a force field to be both accurate and transferable, i.e., useful for a range of similar systems, it must separately reproduce each of the four physical components of the total intermolecular interaction energy, namely, Coulomb, exchange repulsion, induction, and dispersion. In this context, the sum of interacting fragment ab initio (SIBFA) force field, 11 initially tested in the paper of van Duijneveldt-van de Rijdt's et al, was recently refined in order to more closely reproduce each of the individual components of the Hartree-Fock intermolecular interaction energy.…”