We present an implementation of the time-dependent configuration-interaction singles (TDCIS) method for treating atomic strong-field processes. In order to absorb the photoelectron wave packet when it reaches the end of the spatial grid, we add to the exact nonrelativistic many-electron Hamiltonian a radial complex absorbing potential (CAP). We determine the orbitals for the TDCIS calculation by diagonalizing the sum of the Fock operator and the CAP using a flexible pseudospectral grid for the radial degree of freedom and spherical harmonics for the angular degrees of freedom. The CAP is chosen such that the occupied orbitals in the Hartree-Fock ground state remain unaffected. Within TDCIS, the many-electron wave packet is expanded in terms of the Hartree-Fock ground state and its single excitations. The virtual orbitals satisfy nonstandard orthogonality relations, which must be taken into consideration in the calculation of the dipole and Coulomb matrix elements required for the TDCIS equations of motion. We employ a stable propagation scheme derived by second-order finite differencing of the TDCIS equations of motion in the interaction picture and subsequent transformation to the Schrödinger picture. Using the TDCIS wave packet, we calculate the expectation value of the dipole acceleration and the reduced density matrix of the residual ion. The technique implemented will allow one to study electronic channel-coupling effects in strong-field processes.

The ability to predict the pressure dependence of chemical reaction rates would be a great boon to kinetic modeling of processes such as combustion and atmospheric chemistry. This pressure dependence is intimately related to the rate of collision-induced transitions in energy E and angular momentum J. We present a scheme for predicting this pressure dependence based on coupling trajectory-based determinations of moments of the E,J-resolved collisional transfer rates with the two-dimensional master equation. This completely a priori procedure provides a means for proceeding beyond the empiricism of prior work. The requisite microcanonical dissociation rates are obtained from ab initio transition state theory. Predictions for the CH4 = CH3 + H and C2H3 = C2H2 + H reaction systems are in excellent agreement with experiment.

Using full dimensional EOM-IP-CCSD/aug-cc-pVTZ potential energy surfaces, the photoelectron spectrum, vibrational structure, and ionization dynamics of the water dimer radical cation, (H 2 O) + 2 , were computed. We also report an experimental photoelectron spectrum which is derived from photoionization efficiency mea- Hamiltonian was used to compute the ionization dynamics for both the ground state and first excited state of the cation. The dynamics show markedly different behavior and spectroscopic signatures depending on which state of the cation is accessed by the ionization. Ionization to the ground-state cation surface induces a hydrogen transfer which is complete within 50 femtoseconds, whereas ionization to the first excited state results in a much slower process.