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Accurate binding energies of molecules to water clusters are relevant for understanding intermolecular interactions and various chemical applications. They enter models of interstellar chemical processes, as binding to icy grains influences surface reactions and thus affects calculated gas-phase abundances. Unfortunately, many astrochemical molecules (especially radicals and ions) are incompletely characterised in these models. To address this, we report computational searches for optimal structures and benchmark binding and condensation energies for sets of neutral, radical, cationic, and anionic molecules of astrochemical relevance with clusters of $ N=1{\rm -}4 $ N = 1 − 4 water molecules. These calculations utilised reliable density functionals for geometry optimisation, and coupled cluster (CCSD(T)) single point calculations with large basis sets. Four energetic binding motifs (weak, intermediate, strong or covalently bonded) were observed depending on the chemical nature of the guest molecule. Neutral closed and open-shell molecules with strong dipoles and a greater potential for hydrogen bonding are more tightly bound to water clusters compared to non-polar ones. For closed-shell cationic and anionic species, barrier-less reactions with water clusters occur, which reveals radical-free routes to molecular processing in the gas phase and on amorphous ice surfaces. [ABSTRACT FROM AUTHOR]

A full derivation of the analytic transformation of the quadratic, cubic, and quartic force constants from normal coordinates to Cartesian coordinates is given. Previous attempts at this transformation have resulted in non-linear transformations; however, for the first time, a simple linear transformation is presented here. Two different approaches have been formulated and implemented, one of which does not require prior knowledge of the translation-rotation eigenvectors from diagonalization of the Hessian matrix. The validity of this method is tested using two molecules H2O and c-C3H2D+. [ABSTRACT FROM AUTHOR]

The formalism for a relativistic open-shell CCSD(T) method is presented and implemented in a computer program, RELCCSD. The code can be used for calculations with 2- or 4-component relativistic reference wave functions and allows a full inclusion of the spin–orbit coupling. The code is interfaced to the MOLFDIR program system. We illustrate its use with ab initio calculations of the fine structure splittings of Cl, FO, ClO, O+2, and O-2. The triples correction is found to make a large contribution to the Cl atom splitting, which is within 23 cm-1, of the experimental value. The molecular results are within 4 cm-1 of the experimental values where these are available. The value for FO is predicted to be -195±4 cm-1, in good agreement with experiment. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Singles and doubles coupled-cluster (CCSD) theory and the CCSD(T) method, CCSD plus a perturbational estimate of the effects of connected triple excitations, have been used to determine the equilibrium structures, dipole moment, vibrational frequencies, and infrared intensities of the HNO, FNO, and ClNO species. A double polarized triple-zeta (TZ2P) quality basis set was used. The CCSD(T) results are found to be in very good agreement with experiment, indicating that the CCSD(T) method performs well in describing the weak X–NO single bond. The experimental heats of formation of the FNO, and ClNO species are examined for internal consistency with experimental data for FNO2 and ClNO2, by using an isodesmic reaction. It is concluded that the experimental data exhibit a small inconsistency. An approach, involving in addition the use of homodesmic reactions, by which a more consistent set of thermochemical data may be obtained is proposed and discussed. [ABSTRACT FROM AUTHOR]

The lowest 1Σ+ and 3Π states of the BN molecule have been studied using multireference configuration interaction (MRCI) and averaged coupled-pair functional (ACPF) methods and large atomic natural orbital (ANO) basis sets, as well as several coupled cluster methods. Our best calculations strongly support a 3Π ground state, but the a 1Σ+ state lies only 381±100 cm-1 higher. The a 1Σ+ state wave function exhibits strong multireference character and, consequently, the predictions of the perturbationally-based single-reference CCSD(T) coupled cluster method are not as reliable in this case as the multireference results. The best theoretical predictions for the spectroscopic constants of BN are in good agreement with experiment for the X 3Π state, but strongly suggest a misassignment of the fundamental vibrational frequency for the a 1Σ+ state. [ABSTRACT FROM AUTHOR]

Electron correlation energies have been determined for 14 different molecules which represent a range of chemical bonding situations. These have been determined with the coupled-cluster single, double, and triple (CCSDT) excitation model as well as with several coupled-cluster methods that include only an approximate treatment of connected triple excitations, viz. CCSDT-1a, CCSDT-1b, CCSDT-2, CCSDT-3, CCSDT-4, and the recently proposed CCSD(T) method. All of the CCSDT-x methods include the effects of connected triple excitations in an iterative manner, whereas in CCSDT(T) these are included perturbationally. For chemical systems which are well represented by a single-determinant reference function, some of the CCSDT-xmethods (CCSDT-1a, CCSDT-1b, and CCSDT-4) perform marginally better than the CCSD(T) approach in reproducing the CCSDT results. However, as nondynamical correlation becomes more important the good agreement from the CCSDT-x methods deteriorates rapidly, while the error in CCSD(T) remains more consistent. For the 14 molecules considered in this work, the average error of the CCSD(T) method relative to CCSDT (667 μhartrees) is considerably below that obtained from any of the CCSDT-x methods. It is concluded that CCSD(T) is to be preferred over any of the other approximate methods, both because it is the least expensive and also because it is generally the most accurate approximation to CCSDT. [ABSTRACT FROM AUTHOR]