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We present significant algorithmic improvements to a recently-proposed direct quantum dynamics method, based upon combining well established grid-based quantum dynamics approaches and expansions of the potential energy operator in terms of a weighted sum of Gaussian functions. Specifically, using a sum of lowdimensional Gaussian functions to represent the potential energy surface (PES), combined with a secondary fitting of the PES using singular value decomposition, we show how standard grid-based quantum dynamics methods can be dramatically accelerated without loss of accuracy. This is demonstrated by on-the-fly simulations (using both standard grid-based methods and MCTDH) of both proton transfer on the electronic ground state of salicylaldimine and the non-adiabatic dynamics of pyrazine.

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We present a method for performing non-adiabatic, grid-based nuclear quantum dynamics calculations using diabatic potential energy surfaces (PESs) generated "on-the-fly". Gaussian process regression is used to interpolate PESs by using electronic structure energies, calculated at points in configuration space determined by the nuclear dynamics, and diabatising the results using the propagation diabatisation method reported recently [J. Phys. Chem. A, 119, 12457 -12470 (2015)]. To test this new method, the nuclear dynamics on the ground and first electronic excited states of the butatriene cation is studied using a grid-based method. The evolution of diabatic state populations is in very good agreement with those produced using a fitted potential. Overall, our scheme offers a route towards accurate quantum dynamics on diabatic PESs learnt on-the-fly.Keywords: Direct-dynamics, Diabatisation, Grid-based quantum dynamics, Butatriene 2010 MSC: 00-01, 99-00The study of nuclear quantum dynamics of nuclei is of great importance in helping to understand of the time-evolution of molecular systems upon excitation of the electronic degrees-of-freedom (DOFs); such simulations can make direct connections to the states. [6,7] One can model these non-adiabatic transitions using classical mechanics, as in the trajectory surface hopping (TSH) algorithm, [2,[8][9][10] but as the dynamics are inherently quantum mechanical, it is better to use a quantum mechani-25 cal method such as the multi-configuration timedependent Hartree (MCTDH) approach [1, 11,12] where possible.The major bottleneck in performing quantum dynamics calculations is usually not the wavefunc-30 tion time-propagation, but the creation of an appropriate PES on which to run the dynamics. As quantum mechanics is non-local, one needs a PES which is known everywhere in the configuration space of the nuclear motion prior to running the 35 dynamics. For the fully quantum mechanical study of non-adiabatic systems it is usually necessary to convert the PESs from the adiabatic representation to a diabatic representation. The adiabatic representation of the potential is an energy-ordered set 40 of PESs, and corresponds to the energies generated by electronic structure programs. However, at points in configuration space where adiabatic surfaces become degenerate, such as at conical interePreprint submitted to Chemical Physics Letters December 20, 2016 sections (CIs), there is a discontinuity in the gradi-45 ent of the states, such that the adiabatic states are no longer smooth; furthermore, the coupling between the states at these points is also infinite. Neither property of the adiabatic PESs is conducive to performing wavepacket dynamics, so transforma-50 tion to the diabatic representation is performed, resulting in smoothly varying surfaces with finite couplings. Diabatic representations are not unique for a given set of adiabatic states, so an appropriate diabatisation scheme must be chosen before the 55 PES can be used in a dynamics calculation. PESs are usua...

Photoprotection from harmful ultraviolet (UV) radiation exposure is a key problem in modern society. Mycosporine-like amino acids found in fungi, cyanobacteria, macroalgae, phytoplankton, and animals are already presenting a promising form of natural photoprotection in sunscreen formulations. Using time-resolved transient electronic absorption spectroscopy and guided by complementary ab initio calculations, we help to unravel how the core structures of these molecules perform under UV irradiation. Through such detailed insight into the relaxation mechanisms of these ubiquitous molecules, we hope to inspire new thinking in developing next-generation photoprotective molecules.

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.

We describe a method for performing nuclear quantum dynamics calculations using standard, grid-based algorithms, including the multiconfiguration time-dependent Hartree (MCTDH) method, where the potential energy surface (PES) is calculated "on-the-fly". The method of Gaussian process regression (GPR) is used to construct a global representation of the PES using values of the energy at points distributed in molecular configuration space during the course of the wavepacket propagation. We demonstrate this direct dynamics approach for both an analytical PES function describing 3-dimensional proton transfer dynamics in malonaldehyde and for 2- and 6-dimensional quantum dynamics simulations of proton transfer in salicylaldimine. In the case of salicylaldimine we also perform calculations in which the PES is constructed using Hartree-Fock calculations through an interface to an ab initio electronic structure code. In all cases, the results of the quantum dynamics simulations are in excellent agreement with previous simulations of both systems yet do not require prior fitting of a PES at any stage. Our approach (implemented in a development version of the Quantics package) opens a route to performing accurate quantum dynamics simulations via wave function propagation of many-dimensional molecular systems in a direct and efficient manner.

The Stark effect is produced when a static field alters molecular states. When the field applied is time dependent, the process is known as the dynamic Stark effect. Of particular interest for the control of molecular dynamics is the Non-Resonant Dynamic Stark Effect (NRDSE), in which the time dependent field is unable to effect a one-photon excitation. The intermediate strength laser pulse instead shapes the potential energy surfaces (PES) and so guides the evolution of the system. A prototype control scheme uses the NRDSE to change the topography of PES in regions where they intersect, thus providing control over photochemistry. Following earlier experimental work, in this paper we study the NRDSE on a new 3 state model of the IBr molecule to gain insight into the mechanism of control at the avoided crossing that governs the branching ratio of the photodissociation.