QUANTICS: A general purpose package for QUANTum molecular dynamICS simulations

Quantics is a general purpose program package to simulate the time-evolution of a molecular system by solving the time-dependent Schrödinger equation. The main code is based on the multi-configurational time-dependent Hartree (MCTDH) algorithm in various variants, including the powerful multilayer-MCTDH algorithm that has been used to propagate a wavefunction for up to 1000 degrees of freedom. MCTDH uses a contraction of traditional discrete basis set representations of the Hamiltonian and wavefunction, and QUANTICS includes a range of variable representation (DVR) grid basis sets and collocation methods. Input is via ascii text files and for molecules with analytical potential functions no programming is required. A library of potential functions is included to treat more complicated cases, and more functions can be added as required by the user. The code also includes the variational multi-configurational Gaussian (vMCG) method that is based on a Gaussian wavepacket expansion of the wavefunction. vMCG can be run in a “direct” manner (DD-vMCG), calculating the potential energy surfaces on-the-fly using a number of quantum chemistry programs. In addition to wavepacket propagation, Quantics can solve the time-independent Schrödinger equation for small systems and can solve the Liouville–von-Neumann equation to propagate density matrices. The Package includes auxiliary programs to help set up calculations and analyse the output. Quantics is a community code of the UK Collaborative Computational Project for Quantum Dynamics (CCPQ) and the European E-CAM project, an e-infrastructure for software development run by the Centre Européen de Calcul Atomique et Moléculaire (CECAM). Through this it has become a framework for general dynamics codes, for example enabling an external surface hopping code to use the QUANTICS input and operator interfaces. Program summary: Program Title: Quantics Program Files doi: http://dx.doi.org/10.17632/x9dcpc2r5c.1 Licensing provisions: LGPLv3 Programming language: Fortran90. Some Fortran77, Fortran2003, C and python. Nature of problem: Solving the time-dependent Schrödinger equation for a set of nuclei allows a range of physical processes to be studied including all quantum effects. This allows an experimental signal to be given a molecular interpretation. Typical applications are scattering cross-sections or time-resolved spectra, but also rate constants and other transport properties are possible. The exact problem to be solved is defined by the Hamiltonian, which must be provided by the user, and the initial wavepacket, again defined by the user. The final analysis of the evolving wavepacket then provides the experimental signal or molecular property. Solution method: A range of methods are possible for solving the time-evolution of a wavepacket (see main text). These can be broadly described as basis-set methods, in which the wavepacket and Hamiltonian are expanded in a set of functions. Various functions are possible, including grid-based sets (DVRs and collocation), and Gaussian wavepackets. The wavepacket can then be propagated using a variety of algorithms depending on the representation chosen. These include the full numerically-exact solution, various versions of the multi-configurational time-dependent Hartree method and approximate methods such as trajectory surface hopping. Full details are given in the documentation provided with the package and in a book and a number of review articles [1,2,3]. Additional comments including restrictions and unusual features: The code has been tested on a number of linux distributions and compilers. It works best with a bash environment and a gnu gcc / gfortran compiler greater than version 4.8. The code is parallelised in parts using either OpenMP or MPI. There is a suite of test calculations to test an installation. References [1] M. H. Beck, A. Jäckle, G. A. Worth, H.-D. Meyer, The multiconfiguration time-dependent Hartree method: A highly efficient algorithm for propagating wavepackets., Phys. Rep. 324 (2000) 1–105. [2] H.-D. Meyer, G. A. Worth, Quantum molecular dynamics: propagating wavepackets and density operators using the multiconfiguration time-dependent Hartree method, Theo. Chem. Acc. 109 (2003) 251–267. [3] H.-D. Meyer, F. Gatti, G. A. Worth, Multidimensional Quantum Dynamics: MCTDH Theory and Applications, Wiley-VCH, Weinheim, Germany, 2009

Modelling the non-radiative singlet excited state isomerization of diphenyl-acetylene: A vibronic coupling model

Tolane (diphenyl-acetylene) is the smallest component of macromolecular arrays known as dendrimers that have interesting energy transport properties after photo-excitation. In this paper, a vibronic coupling Hamiltonian is set up to describe the initial isomerization of this molecule. The calculated absorption spectrum is in good agreement with experiment, with the ordering of states and energies from MRCI-DFT calculation. The focus of the study is the pathway for photo-excited isomerisation from the linear geometry at the Frank-Condon point to a trans-structure. The model shows that the origin of the excited-state minimum for the trans-isomer is due to stabilisation of a high lying state. Quantum dynamics calculations using the MCTDH algorithm show the model agrees with experiment that isomerisation only occurs at high temperature. It also suggests that internal conversion to the S1 global minima happens via second order coupling terms, which can explain the observed picosecond timescales

On the Intrinsically Low Quantum Yields of Pyrimidine DNA Photodamages: Evaluating the Reactivity of the Corresponding Minimum Energy Crossing Points

The low quantum yield of photoformation of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) adducts in DNA bases is usually associated with the presence of more favorable nonreactive decay paths and with the unlikeliness of exciting the system in a favorable conformation. Here, we prove that the ability of the reactive conical intersection to bring the system either back to the absorbing conformation or to the photoproduct must be considered as a fundamental factor in the low quantum yields of the mentioned photodamage. In support of the proposed model, the one order of magnitude difference in the quantum yield of formation of the cyclobutane thymine dimer with respect to the thymine-thymine (6-4) adduct is rationalized here by comparing the reactive ability of the seam of intersections leading respectively to the cyclobutane thymine dimer and the oxetane precursor of the thymine-thymine (6-4) adduct at the CASPT2 level of theory

Vibronic coupling model to calculate the photoelectron spectrum of phenol

The photoelectron spectrum for the two lowest ionisation bands of phenol has been simulated using quantum dynamic methods. A vibronic coupling Hamiltonian was set up consisting of seven vibrational modes and the first two ionised states. Parameters for the model are obtained by fitting adiabatic surfaces to a series of points calculated using ab initio methods. Such a model allows non-adiabatic couplings between the states to be included. CASSCF calculations used in this work provide reliable quantum chemical information for the model and the calculated photoelectron spectrum shows good agreement to experiment. The vibrational fine structure of both bands are reassessed and different assignments to those previously reported are detailed. The existence of a conical intersection between the ionised states is reported and its role in the dynamics of phenol upon ionisation is examined

Direct nonadiabatic quantum dynamics simulations of the photodissociation of phenol

Gaussian wavepacket methods are becoming popular for the investigation of nonadiabatic molecular dynamics. In the present work, a recently developed efficient algorithm for the Direct Dynamics variational Multi-Configurational Gaussian (DD-vMCG) method has been used to describe the multidimensional photodissociation dynamics of phenol including all degrees of freedom. Full-dimensional quantum dynamic calculations including for the first time six electronic states (1ππ, 11ππ*, 11πσ*, 21πσ*, 21ππ*, 31ππ*), along with a comparison to an existing analytical 4-state model for the potential energy surfaces are presented. Including the fifth singlet excited state is shown to have a significant effect on the nonadiabatic photodissociation of phenol to the phenoxyl radical and hydrogen atom. State population and flux analysis from the DD-vMCG simulations of phenol provided further insights into the decay mechanism, confirming the idea of rapid relaxation to the ground state through the 1ππ/11πσ* conical intersection

Micro-solvated DMABN: Excited state quantum dynamics and dual fluorescence spectra

In this work, we report a complete analysis by theoretical and spectroscopic methods of the short-time behaviour of 4-(dimethylamino)benzonitrile (DMABN) in the gas phase as well as in cyclohexane, tetrahydrofuran, acetonitrile, and water solution, after excitation to the La state. The spectroscopic properties of DMABN were investigated experimentally using UV absorption and fluorescence emission spectroscopy. The computational study was developed at different electronic structure levels and using the Polarisable Continuum Model (PCM) and explicit solvent molecules to reproduce the solvent environment. Additionally, excited state quantum dynamics simulations in the diabatic picture using the direct dynamics variational multiconfigurational Gaussian (DD-vMCG) method were performed, the largest quantum dynamics “on-the-fly” simulations performed with this method until now. The comparison with fully converged multilayer multiconfigurational time-dependent Hartree (ML-MCTDH) dynamics on parametrised linear vibronic coupling (LVC) potentials show very similar population decays and evolution of the nuclear wavepacket. The ring C=C stretching and three methyl tilting modes are identified as the responsible motions for the internal conversion from the La to the Lb states. No major differences are observed in the ultrafast initial decay in different solvents, but we show that this effect depends strongly on the level of electronic structure used

Improved algorithm for the direct dynamics variational multi-configurational Gaussian method.

The Direct Dynamics variational Multi-Configurational Gaussian (DD-vMCG) method provides a fully quantum mechanical solution to the time-dependent Schrödinger equation for the time evolution of nuclei with potential surfaces calculated on-the-fly using a quantum chemistry program. Initial studies have shown its potential for flexible and accurate simulations of non-adiabatic excited-state molecular dynamics. In this paper, we present developments to the DD-vMCG algorithm that improve both its accuracy and efficiency. First, a new, efficient parallel algorithm to control the DD-vMCG database of quantum chemistry points is presented along with improvements to the Shepard interpolation scheme. Second, the use of symmetry in describing the potential surfaces is introduced along with a new phase convention in the propagation diabatization. Benchmark calculations on the allene radical cation including all degrees of freedom then show that the new scheme is able to produce a consistent non-adiabatic coupling vector field. This new DD-vMCG version thus opens the route for effectively and accurately treating complex chemical systems using quantum dynamics simulations

Multi-layer Gaussian-based multi-configuration time-dependent Hartree (ML-GMCTDH) simulations of ultrafast charge separation in a donor–acceptor complex

We report on first applications of the Multi-Layer Gaussian-based Multi-Configuration Time-Dependent Hartree (ML-GMCTDH) method [Römer et al., J. Chem. Phys. 138, 064106 (2013)] beyond its basic two-layer variant. The ML-GMCTDH scheme provides an embedding of a variationally evolving Gaussian wavepacket basis into a hierarchical tensor representation of the wavefunction. A first-principles parameterized model Hamiltonian for ultrafast non-adiabatic dynamics in an oligothiophene–fullerene charge transfer complex is employed, relying on a two-state linear vibronic coupling model that combines a distribution of tuning type modes with an intermolecular coordinate that also modulates the electronic coupling. Efficient ML-GMCTDH simulations are carried out for up to 300 vibrational modes using an implementation within the QUANTICS program. Excellent agreement with reference ML-MCTDH calculations is obtained

Electron transfer in photoexcited pyrrole dimers

Following on from previous experimental and theoretical work [Neville et al., Nat. Commun. 7, 11357 (2016)], we report the results of a combined electronic structure theory and quantum dynamics study of the excited state dynamics of the pyrrole dimer following excitation to its first two excited states. Employing an exciton-based analysis of the Ã(π3s/σ*) and ˜ B (π3s/3p/σ*) states, we identify an excited-state electron transfer pathway involving the coupling of the Ã(π3s/σ*) and ˜ B (π3s/3p/σ*) states and driven by N–H dissociation in the ˜ B (π3s/3p/σ*) state. This electron transfer mechanism is found to be mediated by vibronic coupling of the ˜ B state, which has a mixed π3s/3p Rydberg character at the Franck-Condon point, to a high-lying charge transfer state of the πσ* character by the N–H stretch coordinate. Motivated by these results, quantum dynamics simulations of the excited-state dynamics of the pyrrole dimer are performed using the multiconfigurational time-dependent Hartree method and a newly developed model Hamiltonian. It is predicted that the newly identified electron transfer pathway will be open following excitation to both the Ã(π3s/σ*) and ˜ B (π3s/3p/σ*) states and may be the dominant relaxation pathway in the latter case

Unlocking the Double Bond in Protonated Schiff Bases by Coherent Superposition of S_{1} and S_{2}

The primary event occurring during the E-to-Z photoisomerization reaction of retinal protonated Schiff base (rPSB) is single-to-double bond inversion. In this work we examine the nuclear dynamics that occurs when the initial excited state is a superposition of the S_{1} and S_{2} electronic excited states that might be created in a laser experiment. The nuclear dynamics is dominated by double bond inversion that is parallel to the derivative coupling vector of S_{1} and S_{2}. Thus, the molecule behaves as if it were at a conical intersection even if the states are nondegenerate