Sotiris S. Xantheas

On the importance of the fragment relaxation energy terms in the estimation of the basis set superposition error correction to the intermolecular interaction energy

The inclusion of the fragment relaxation energy terms in the estimation of the basis set superposition error (BSSE) correction to the interaction energy is necessary in order to ensure formal convergence to the uncorrected result at the complete basis set (CBS) limit. The problems associated with their omission are demonstrated for F−(H2O), Cl−(H2O), and (H2O)2 especially when very large basis sets are used. The family of correlation consistent basis sets allows for a heuristic extrapolation of both uncorrected and BSSE‐corrected electronic energy differences of the three complexes to the MP2 CBS limits of −27.1, −15.1, and −4.9 kcal/mol respectively.

Cooperativity and hydrogen bonding network in water clusters

Abstract Clusters of water molecules are held together by hydrogen bonding networks. These networks are differentiated by the participation of the individual water molecules in the hydrogen bonds either as proton donors (d), proton acceptors (a) or their combinations. It has long been assumed that the stability of clusters is determined by the dominant two-body interactions between the water molecules. We have found that homodromic hydrogen bonding networks, i.e. those exhibiting donor–acceptor (da) arrangements between all water molecules, are associated with the largest non-additivities among other networks present in low lying minima of small water clusters. This finding offers a novel explanation for the stability of homodromic rings that are the global minima for the clusters trimer through pentamer. Among the non-additive terms, three-body terms are mainly responsible for determining the relative stabilities between the various trimer through pentamer isomers. This suggests that purely two-body pairwise additive potentials will result in errors exceeding 20% for clusters larger than the pentamer. Among all higher order components, the three-body term was found to be the most important.

Development of transferable interaction models for water. II. Accurate energetics of the first few water clusters from first principles

We report accurate energetics for the water trimer through pentamer global ring minima and four low-lying hexamer isomers (cage, prism, book, cyclic S6) from first principles electronic structure calculations. The family of augmented correlation-consistent orbital basis sets of double through quintuple zeta quality was used in order to estimate complete basis set (CBS) limits for the cluster total association energies at the second order perturbation (MP2) level of theory. These are −15.8 kcal/mol (trimer), −27.6 kcal/mol (tetramer), −36.3 kcal/mol (pentamer), −45.9 kcal/mol (prism hexamer), −45.8 kcal/mol (cage hexamer), −45.6 kcal/mol (book hexamer), and −44.8 kcal/mol (ring S6 hexamer). Effects of higher correlation, estimated at the coupled cluster plus single and double with a perturbative estimate of the triple excitations [CCSD(T)] level of theory, as well as inclusion of estimates for core–valence correlation suggest that these estimates are accurate to within 0.2 kcal/mol.

Ab initio studies of cyclic water clusters (H2O)n, n=1–6. III. Comparison of density functional with MP2 results

The optimal structures and harmonic vibrational frequencies of ring water clusters (H2O)n, n=1–6 are computed using density functional theory (DFT). The exchange functionals of Slater (S), Becke (B), the correlation functionals of Lee–Yang–Parr (LYP), Vosko–Wilk–Nusair (VWN), Perdew’s local (PL), and gradient‐corrected (P86) as well as their combinations are used to perform benchmark calculations on the water monomer and dimer. We use the augmented correlation‐consistent polarized valence orbital basis set of double (aug‐cc‐pVDZ) and triple zeta quality (aug‐cc‐pVTZ) in order to compare the DFT with earlier MP2 results [J. Chem. Phys. 99, 8774 (1993); 100, 7523 (1994)]. Better overall agreement with the MP2 and experimental results for the water monomer and dimer is observed for the B–LYP and B–P86 functional combinations. The optimal structures, harmonic vibrational frequencies, and energetics of the clusters trimer through hexamer are computed at the B–LYP/aug‐cc‐pVDZ level of theory. This functional co...

THE PARAMETRIZATION OF A THOLE-TYPE ALL-ATOM POLARIZABLE WATER MODEL FROM FIRST PRINCIPLES AND ITS APPLICATION TO THE STUDY OF WATER CLUSTERS (N=2-21) AND THE PHONON SPECTRUM OF ICE IH

We present the parametrization of a new polarizable model for water based on Thole’s method [Chem. Phys. 59, 341 (1981)] for predicting molecular polarizabilities using smeared charges and dipoles. The potential is parametrized using first principles ab initio data for the water dimer. Initial benchmarks of the new model include the investigation of the properties of water clusters (n=2–21) and (hexagonal) ice Ih using molecular dynamics simulations. The potential produces energies and nearest-neighbor (H-bonded) oxygen–oxygen distances that agree well with the ab initio results for the small water clusters. The properties of larger clusters with 9–21 water molecules using predicted structures from Wales et al. [Chem. Phys. Lett. 286, 65 (1998)] were also studied in order to identify trends and convergence of structural and electric properties with cluster size. The simulation of ice Ih produces a lattice energy of −65.19 kJ/mol (expt. −58.9 kJ/mol) with an average dipole moment of 2.849 D. The calculated...

Development of transferable interaction models for water. IV. A flexible, all-atom polarizable potential (TTM2-F) based on geometry dependent charges derived from an ab initio monomer dipole moment surface

In this work we examine the consequences of incorporating the ab initio derived monomer potential-energy surface and nonlinear dipole surface of Partridge and Schwenke [J. Chem. Phys. 106, 4618 (1997)] into the previously developed TTM2-R model of Burnham et al. [J. Chem. Phys. 116, 1500 (2002)] in order to develop a new, all-atom polarizable, flexible model for water (TTM2-F). We found that the use of the nonlinear dipole surface is essential in modeling the change in the internal geometry of interacting water molecules and, in particular, the increase in the internal H–O–H bend angle with cluster size. This is the first demonstration of a flexible model which shows an increase in the bending angle in clusters. An explanation for this behavior is presented using the concept of geometric polarizabilities. The model furthermore reproduces the n=2–6 cluster binding energies to within an RMS deviation of 0.05 kcal/mol per hydrogen bond with respect to the MP2 complete basis set estimates. Preliminary results...

Microscopic hydration of the fluoride anion

A combined experimental and theoretical investigation of the step-wise hydration of the fluoride ion has been performed in order to characterize the details of its solvation at the microscopic level. The comparable anion–water and water–water interactions pose a challenging experimental/theoretical problem due to competing intermolecular forces in these small ionic clusters. Vibrational spectra of size-selected F−(H2O)3−5 in the O–H stretching region, coupled with high level ab initio calculations, have been used to analyze the spectra and assign the structures of these species. The interaction between the fluoride anion and water plays the dominant role, resulting in internally solvated clusters. The microhydration of fluoride ion is thus qualitatively different from the other halide ions.

Molecular multipole moments of water molecules in ice Ih

We have used an induction model including dipole, dipole–quadrupole, quadrupole–quadrupole polarizability and first hyperpolarizability as well as fixed octopole and hexadecapole moments to study the electric field in ice. The self-consistent induction calculations gave an average total dipole moment of 3.09 D, a 67% increase over the dipole moment of an isolated water molecule. A previous, more approximate induction model study by Coulson and Eisenberg [Proc. R. Soc. Lond. A 291, 445 (1966)] suggested a significantly smaller average value of 2.6 D. This value has been used extensively in recent years as a reference point in the development of various polarizable interaction potentials for water as well as for assessment of the convergence of water cluster properties to those of bulk. The reason for this difference is not due to approximations made in the computational scheme of Coulson and Eisenberg but rather due to the use of less accurate values for the molecular multipoles in these earlier calculations.

The formation of cyclic water complexes by sequential ring insertion: Experiment and theory

The growth of water clusters in liquid helium droplets results in the formation of cyclic structures up to and including the hexamer. In view of the sequential nature of the molecular pick-up process, the formation of water rings involves the insertion of water monomers into preformed cyclic water clusters. The implication of this observation is that the barriers to the ring insertion process are low enough to be overcome during the experiment. This paper presents a combined experimental and theoretical effort to explore the insertion process in detail. Our results provide important new insights into the dynamics of hydrogen-bonded networks. We map out the cluster potential energy surfaces and visualize them using disconnectivity graphs. Nonequilibrium walks on these surfaces show that ring water clusters can be formed during sequential addition of water molecules by surmounting small barriers that are thermally accessible even at the low temperature of the experiment. We find that the effects of zero-poi...

Development of transferable interaction models for water. I. Prominent features of the water dimer potential energy surface

We present an analysis of the morphology of the water dimer potential energy surface (PES) obtained from ab initio electronic structure calculations and perform a quantitative comparison with the results from various water potentials. In order to characterize the morphology of the PES we have obtained minimum energy paths (MEPs) as a function of the intermolecular O–O separation by performing constrained optimizations under various symmetries (Cs, Ci, C2, and C2v). These constitute a primitive map of the dimer PES and aid in providing an account for some of its salient features such as the energetic stabilization of “doubly hydrogen-bonded” configurations for R(O–O)<2.66 A. Among the various interaction potentials that are examined, it is found that the family of anisotropic site potential (ASP) models agrees better with the ab initio results in reproducing the geometries along the symmetry-constrained MEPs. It is demonstrated that the models that produce closest agreement with the morphology of the ab in...