A method for diabatising multiple electronic states on-the-fly within the direct dynamics variational multi-configuration Gaussian method for calculating quantum nuclear dynamics is presented. The method is based upon the propagation of the adiabatic–diabatic transformation matrix along the paths followed by the Gaussian basis functions that constitute the nuclear wave function, by use of a well-known differential equation relating the matrix and the nonadiabatic vector coupling terms between the electronic states. The implementation of the method is described, and test calculations are presented using the ground and first-excited states of the butatriene cation as an example, allowing comparison to the earlier regularisation diabatisation scheme as well as to full nuclear dynamics on a precomputed potential energy surface. The new scheme is termed propagation diabatisation.

A model Hamiltonian based on the vibronic coupling model is developed to describe the excited state dynamics of 3-pyrroline. With the use of the method of improved relaxation in conjunction with the MCTDH wavepacket propagation algorithm, vibrational eigenstates corresponding to both the axial and equatorial conformers of 3-pyrroline are calculated and subsequently used in a conformer-resolved study of the photodissociation of 3-pyrroline following excitation to its S1(3s/πσ*) and S2(3px) states. In analogy with ammonia, the excited state dynamics of both conformers of 3-pyrroline are found to be dominated by the (quasi-) planarization of the molecule in its electronically excited states and predominantly diabatic behavior of dissociation mediated by a conical intersection between the S1 and S0 states.

A model Hamiltonian based on the quadratic vibronic coupling model is developed to describe the photoinduced dynamics of aniline excited to the manifold of states comprising its first six singlet electronic states. The model Hamiltonian is parametrized by fitting to the results of extensive EOM-CCSD calculations and its validity tested through the calculation of the first two bands in the electronic absorption spectrum of aniline. It is found that two previously neglected 3p Rydberg states play an important role in the dynamics of aniline following excitation into the first two 1ππ* states. Assignments of the vibrational structure seen in the experimental spectrum is made, and the role played by the Herzberg–Teller effect in excitation to the first 1ππ* state is analyzed.

Control of the photodissociation of ammonia, by the nonresonant dynamic Stark effect, has been studied theoretically by the numerically exact propagation of wavepackets on ab initio potential energy surfaces. An assessment of the feasibility of controlling the proportion of the wavepacket which dissociates to produce ground or electronically excited state NH2 fragments, mediated by a conical intersection, has been made by use of a simple two-dimensional, two-state model. It was found that modest control was possible for nonresonant pulses applied during and after excitation, and that the control was caused not by alteration of the position or nature of the conical intersection but by modification of the energy surfaces around the Franck–Condon region. This is made possible by the predissociative nature of the mechanism for hydrogen ejection. The control effect is frequency independent but dependent on pulse, i.e., electric field, strength, indicating that it is indeed due to the Stark effect. Analysis of the control is, however, complicated by the presence of vibrational effects which can come into play if the control pulse frequency is not carefully chosen. By systematically varying the excitation energy, it was also found that the capacity for control is only significant at low energies.

The theory and computation of optical control has been developed over the last 25 years and is now a mature field of research. Initial work provided pictures of how control using light fields in simple systems may be achieved, for example using multiple excitation pathways or pulse sequences. The development of optimal control theory then provided a general method for guiding a system to its target using a shaped laser pulse. Combined with quantum dynamics simulations this has become a widely used tool, and has been applied to a range of systems to show what can be controlled. The present challenge is to gain more insight into the mechanism of control. In addition, methods need to be extended to reach the size of system of interest to technology. In this perspective article we shall give a brief overview of present capabilities and some of the recent developments in quantum dynamics and control simulations.

In this work, we investigate general mechanistic principles that control reaction selectivity following S1/S0 internal conversion in benzene. A systematic relationship is drawn between the varying topology of an extended seam of conical intersection and the balance between two competitive radiationless decay channels: photophysical (benzene reactant regeneration) and photochemical (prefulvene product formation). This is supported by a model quantum dynamics study, using a direct dynamics approach based on variational multiconfiguration Gaussian wavepackets, where initial excitation of specific vibrational modes is designed to generate dynamical pathways that reach selected targets regions of the seam. High-energy regions of the seam are found to be sloped and in favor of the photophysical channel, while lower-energy regions are peaked and give access to the photochemical channel. This changeover could in principle be exploited to define targets for optimal control, by exciting different combinations of specific vibronic levels in S1, accessing different regions of the seam, and giving different products.

We present here a direct quantum dynamics method using variational multi-configuration Gaussian wavepackets. Based on the efficient multi-configuration time-dependent Hartree wavepacket propagation algorithm, it uses on-the-fly quantum chemical calculation of the potential energy and its derivatives rather than fitted surfaces. Intermediate results are stored in a database so that expensive quantum chemical computations can be recycled. This method is intended to treat quantum effects in the photochemistry of large molecules and the use of Cartesian coordinates to perform direct dynamics is discussed with a comparison between Cartesian coordinates of Jacobi vectors and Cartesian coordinates of the nuclei, using various free and constrained approaches depending on the way the rotation is treated. As a test calculation to be compared to full quantum dynamics it is applied here to the computation of the photodissociation spectrum of nitrosyl chloride (NOCl).

It is extremely difficult to execute full quantum dynamics calculations on complex systems (more then three degrees of freedom) due to the exponential increase in computer resources required by these methods. Classical mechanics simulations do not suffer from this problem, but are unable to treat quantum mechanical phenomena such as non-adiabatic effects, which often play a vital role in photochemical processes. A method has been implemented for carrying out dynamical calculations using quasi-classical theory. The time dependent Schrödinger equation is solved using a swarm of trajectories treated under Newtonian laws and Tully's fewest switches trajectory surface hopping is applied to implement the surface switches. The method was applied to ozone, looking at the photodissociation that takes place after excitation into the Chappuis band of the absorption spectrum. While the goal is to treat larger systems, comparison can be made for ozone with numerically exact wavepacket calculations. The method proved successful at calculating quantities such as the rate of population transfer, but there were discrepancies in the details, especially when surface switching occurred from the lower state.

Direct quantum dynamics using variational multi-configuration Gaussian wavepackets is intended to treat quantum effects in the photochemistry of large molecules. It uses on-the-fly quantum chemical calculation of the potential energy and its derivatives rather than fitted surfaces. Intermediate results are stored in a database to avoid repeated quantum chemical computations. The use of Cartesian coordinates is discussed and a comparison is made between free and constrained approaches with respect to rotation. This method is then applied to the computation of the photodissociation spectrum of nitrosyl chloride (NOCl) – a benchmark to be compared to published full quantum calculations.We present a new direct dynamics implementation of the variational multi-configuration Gaussian wavepacket method. The photodissociation spectrum of NOCl is obtained from an initial wavepacket expressed as a function of Cartesian coordinates constrained to separate the rotation. A complete active space self-consistent field potential energy surface is locally calculated on-the-fly.

•Symmetrised polynomials up to 6th order generated for Abelian and non-Abelian point groups.•Polynomials suitable for Hamiltonians describing Renner–Teller and Jahn–Teller systems.•Absorption spectrum calculated for acetylene showing the importance of the Renner–Teller coupling.The vibronic coupling Hamiltonian is a standard model used to describe the potential energy surfaces of systems in which non-adiabatic coupling is a key feature. This includes Jahn–Teller and Renner–Teller systems. The model approximates diabatic potential energy functions as polynomials expanded about a point of high symmetry. One must ensure the model Hamiltonian belongs to the totally symmetric irreducible representation of this point group. Here, a simple approach is presented to generate functions that form a basis for totally symmetric irreducible representations of non-Abelian groups and apply it to D∞h (2D) and O (3D). For the O group, the use of a well known basis-generating operator is also required. The functions generated for D∞h are then used to construct a ten state, four coordinate model of acetylene. The calculated absorption spectrum is compared to the experimental spectrum to serve as a validation of the approach.Figure optionsDownload full-size imageDownload as PowerPoint slide

We present here a direct quantum dynamics method using variational multi-configuration Gaussian wavepackets. Based on the efficient multi-configuration time-dependent Hartree wavepacket propagation algorithm, it uses on-the-fly quantum chemical calculation of the potential energy and its derivatives rather than fitted surfaces. Intermediate results are stored in a database so that expensive quantum chemical computations can be recycled. This method is intended to treat quantum effects in the photochemistry of large molecules and the use of Cartesian coordinates to perform direct dynamics is discussed with a comparison between Cartesian coordinates of Jacobi vectors and Cartesian coordinates of the nuclei, using various free and constrained approaches depending on the way the rotation is treated. As a test calculation to be compared to full quantum dynamics it is applied here to the computation of the photodissociation spectrum of nitrosyl chloride (NOCl).

The theory and computation of optical control has been developed over the last 25 years and is now a mature field of research. Initial work provided pictures of how control using light fields in simple systems may be achieved, for example using multiple excitation pathways or pulse sequences. The development of optimal control theory then provided a general method for guiding a system to its target using a shaped laser pulse. Combined with quantum dynamics simulations this has become a widely used tool, and has been applied to a range of systems to show what can be controlled. The present challenge is to gain more insight into the mechanism of control. In addition, methods need to be extended to reach the size of system of interest to technology. In this perspective article we shall give a brief overview of present capabilities and some of the recent developments in quantum dynamics and control simulations.