Ab initio multiple spawning

Abstract: The Born–Oppenheimer approximation underlies much of chemical simulation and provides the framework defining the potential energy surfaces that are used for much of our pictorial understanding of chemical phenomena However, this approximation breaks down when the dynamics of molecules in excited electronic states are considered Describing dynamics when the Born–Oppenheimer approximation breaks down requires a quantum mechanical description of the nuclei Chemical reaction dynamics on excited electronic states is critical for many applications in renewable energy, chemical synthesis, and bioimaging Furthermore, it is necessary in order to connect with many ultrafast pump–probe spectroscopic experiments In this review, we provide an overview of methods that can describe nonadiabatic dynamics, with emphasis on those that are able to simultaneously address the quantum mechanics of both electrons and nuclei Such ab initio quantum molecular dynamics methods solve the electronic Schrodinger equation alongsi

Abstract: We present a semiclassical surface-hopping method which is able to treat arbitrary couplings in molecular systems including all degrees of freedom. A reformulation of the standard surface-hopping scheme in terms of a unitary transformation matrix allows for the description of interactions like spin−orbit coupling or transitions induced by laser fields. The accuracy of our method is demonstrated in two systems. The first one, consisting of two model electronic states, validates the semiclassical approach in the presence of an electric field. In the second one, the dynamics in the IBr molecule in the presence of spin−orbit coupling after laser excitation is investigated. Due to an avoided crossing that originates from spin−orbit coupling, IBr dissociates into two channels: I + Br(2P3/2) and I + Br*(2P1/2). In both systems, the obtained results are in very good agreement with those calculated from exact quantum dynamical simulations.

Abstract: In a recent paper (G. Worth, P. Hunt and M. Robb, J. Phys. Chem. A, 2003, 107, 621), we used surface hopping direct dynamics calculations to study the molecular dynamics of the butatriene radical cation in the /A manifold, which is coupled by a conical intersection. Here, we present the first direct dynamics calculations using a novel algorithm, again using this ideal test system. The algorithm, which is based on the powerful multi-configuration time-dependent Hartree (MCTDH) wavepacket propagation method, uses a variational basis of coupled frozen Gaussian functions that optimally represent the evolving nuclear wavepacket at all times. Each Gaussian function follows a “quantum trajectory”, along which the potential surface is evaluated by quantum chemistry calculations. As far fewer Gaussian functions are needed than classical trajectories in a semi-classical method, the number of quantum chemical calculations is drastically reduced. A crucial point in direct dynamics. To validate the method, initial calculations have been made using an analytic model Hamiltonian, where it is shown to reproduce the main features of the state population transfer with 8–16 basis functions per state. Coupled to the GAUSSIAN quantum chemistry program, the method is then shown to provide a feasible direct dynamics algorithm for the description of this non-adiabatic process.

Abstract: The ab initio multiple spawning (AIMS) method has been developed to solve the electronic and nuclear Schrodinger equations simultaneously for application to photochemical reaction dynamics. We discuss some details of the implementation of AIMS in the M olpro program package. A few aspects of the implementation are highlighted, including a new multiple timescale integrator and a scheme for solving the coupled-perturbed multiconfiguration self-consistent field (CP-MCSCF) equations in the context of ab initio molecular dynamics. The implementation is very efficient and we demonstrate calculations on the photoisomerization of ethylene using more than 5000 trajectory basis functions. We have included the capability for hybrid quantum mechanics/molecular mechanics (QM/MM) simulations within AIMS, and we investigate the role of an argon solvent in the photoisomerization of ethylene. Somewhat surprisingly, the surrounding argon has little effect on the timescale of non-adiabatic quenching in ethylene.