Photodissociation of water in crystalline ice: a molecular dynamics study

Ultraviolet irradiation of ice is of great interest for understanding the chemistry in both atmospheric and astrophysical environments. In interstellar space, photodissociation of H2O molecules can be a driving force behind the chemistry on icy dust grains in dense, cold molecular clouds even though the flux of UV photons is extremely low. The mechanisms of such photoinduced processes are poorly understood, however. In this work the photodissociation dynamics of a water molecule in crystalline ice at 10 K is studied computationally using classical molecular dynamics. Photodissociation in the first bilayer leads mainly to H atoms desorbing (65%), while in the third bilayer trapping of H and OH dominates (51%). The kinetic energy distribution of the desorbing H atoms is much broader than that for the corresponding gas-phase photodissociation. The H atoms on average move 11 Angstroms before becoming trapped, while OH radicals typically move 2 Angstroms. In accordance with experiments a blueshift of the absorption spectrum is obtained relative to gas-phase water.Comment: 23 pages, 5 figure

The Dalton quantum chemistry program system

Dalton is a powerful general\u2010purpose program system for the study of molecular electronic structure at the Hartree\u2013Fock, Kohn\u2013Sham, multiconfigurational self\u2010consistent\u2010field, M\uf8ller\u2013Plesset, configuration\u2010interaction, and coupled\u2010cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic\u2010structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge\u2010origin\u2010invariant manner. Frequency\u2010dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one\u2010, two\u2010, and three\u2010photon processes. Environmental effects may be included using various dielectric\u2010medium and quantum\u2010mechanics/molecular\u2010mechanics models. Large molecules may be studied using linear\u2010scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platform

The QM/MM approach for wavefunctions, energies and response functions within self-consistent field and coupled cluster theories

This paper presents the coupled cluster/molecular mechanics (CC/MM) and self-consistent field/molecular mechanics (SCF/MM) approaches for wavefunctions, energies and response properties. Two physically different theories are derived, the mean-field and the direct-field interaction approaches, together with expressions for the optimization condition of both variational and non-variational wavefunctions and energies. Also derived are the linear response functions at the CC/MM and SCF/MM levels of theory, and the expressions are compared with the vacuum response functions

Dipole and quadrupole moments of liquid water calculated within the coupled cluster/molecular mechanics method

We present the first study of dipole and quadrupole moments of liquid water calculated using coupled cluster/molecular mechanics (COMM) methods. CUMM methods are used to calculate the total dipole moment of the water dimer and the results are compared to the corresponding ab initio quantum mechanical calculations. For liquid water we find that the introduction of polarization effects are very important for an accurate determination of dipole and quadrupole moments. Furthermore, we find that neglecting the correlation effects in the quantum mechanical part of the system leads to an overestimation of the interaction between the two sub-systems