From anharmonicity to Nuclear Quantum Effects in medium and large sized molecular systems

The nuclear vibrational motion in molecular systems and its spectroscopic representation can be accounted for at different levels of accuracy. The most simple and intuitive one is the diagonalization of the mass-scaled Hessian matrix at the equilibrium geometry. This approach does not take into account the anharmonic part of the Potential Energy Surface (PES), which strongly deviates from the harmonic approximation, expecially for floppy systems. One way to overcome this issue is to run Molecular Dynamics (MD) simulations and calculate the power spectrum as the Fourier-Transform (FT) of the classical velocity autocorrelation function (Cvv). However, in this case the result depends on the choice of the employed trajectories and it does not include nuclear quantum effects. In this work, we examine different choices of initial conditions and how these choices reflect on the computation and the result of the classical spectrum. Furthermore, we show how the inclusion of Nuclear Quantum Effects (NQEs) to the computation of the vibrational eigenvalues contribute to the final results. To obtain these results we implemented the Divide and Conquer Semiclassical Initial Value Representation (DC SCIVR) which made possible to calculate the quantum mechanical spectrum of an explicitly solvated supramolecular system featuring more than 30000 degrees of freedom.