Modeling Photodissociation: Quantum Dynamics Simulations of Methanol

A comprehensive computational study of the gas-phase photodissociation dynamics of methanol is presented. Using a multiconfigurational active space based method (RASSCF) to obtain multidimensional potential energy surfaces (PESs) on-the-fly, direct quantum dynamics simulations were run using the variational multi-configurational Gaussian method (DD-vMCG). Different initial excitation energies were simulated to investigate the dependence of the branching ratios on the electronic state being populated. A detailed mechanistic explanation is provided for the observed differences with respect to the excitation energy. Population of the lowest lying excited state of methanol leads to rapid hydroxyl hydrogen loss as the main dissociation channel. This is rationalized by the strongly dissociative nature of the PES cut along the O–H stretching coordinate, confirmed by the broad feature in the absorption spectrum. In contrast, more energetic excitations lead mainly to C–O bond breaking. Again, analysis of the diabatic surfaces offers a clear explanation in terms of the nature of the electronic states involved and the coupling between them. The type of calculations presented, as well as the subsequent analysis of the results, should be seen as a general workflow for the modeling of photochemical reactions.


S3 Absorption Spectrum
The spectrum presented here is the result of a vMCG calculation using a linear vibronic coupling (LVC) Hamiltonian, parameterised in 4 dimensions.This parameterisation is possible by representing the Hamiltonian as a Taylor series, expressed in normal mode coordinates.At the equilibrium geometry, around which the Taylor series is expanded, the diabatic and adiabatic representations of the potentials is equivalent and hence the following separation is possible: where H 0 and W 0 are harmonic oscillators, representing the ground and excited electronic states, respectively.The oscillator is defined based on the frequencies of the normal modes of the molecule, ω α as follows: and W 0 is simply shifted up by the excitation energy.The gradients and couplings are then provided by the on-and off-diagonal elements of the first order W matrix, respectively.These are commonly expressed as κ parameters for each state i, and λ parameters between each pair of states i and j: The wavefunction is constructed from this LVC model and is propagated using the vMCG EoMs with 256 GWPs.Finally, the spectrum is calculated from the Fourier transform of the autocorrelation function: (5) The parameters for the LVC model are provided in the form of a Quantics operator file, along with the input files for the calculation, as part of the associated datasets to be found at DOI 10.5522/04/25913125 6.5

S5 Direct Dynamics PESs
To show the correct calculation of the relevant surfaces generated during the DD-vMCG simulations, the diabatic and adiabatic surfaces are plotted in Fig. S6.These surfaces are generated from just over 1,500 quantum chemistry calculations, stored in a database (to be found at DOI 10.5522/04/25913125).Cuts along pure normal modes are obtained by setting all other coordinates to zero, apart from the modes of interest.Apart from some "unnatural" kinks, the surfaces adequately describe the regions that are important for dissociation, as well as the other normal modes of methanol.The diabatic surfaces clearly show the crossing between the states, leading to the important conical intersections that play a role in determining the branching ratios.

Figure S1 :
Figure S1: Geometry of methanol in C s symmetry.

Figure S4 :Figure S5 :
FigureS4: Calculated absorption spectrum of methanol obtained from a vMCG calculation, employing a reduced dimensionality linear vibronic coupling Hamiltonian.A horizontal shift of -0.36 eV is applied to the calculated spectrum to correspond with the experimental one.

Figure S6 :
Figure S6: Cuts of the PES constructed from the database points generated during DD-vMCG simulations.Adiabatic (a and b) and diabatic (c and d) surfaces for the O-H stretching and C-O stretching + bending modes.All other modes are at zero.Note: Only the lowest 5 electronic states are shown, however a total of 8 states is calculated and included in the dynamics.

Table S1 :
Cartesian coordinates (in Å) for methanol used in this work, obtained after optimisation at CCSD/aug-cc-pVDZ level of theory.