## November Module of the Month: PerGauss, Periodic Boundary Conditions for gaussian bases

### Description

The module PerGauss (**Per** iodic **Gauss** ians) consists on an implementation of periodic boundary conditions for gaussian bases for the Quantics program package.

In quantum dynamics, the choice of coordinates is crucial to obtain meaningful results. While xyz or normal mode coordinates are linear and do not need a periodical treatment, particular angles, such as dihedrals, must be included to describe accurately the (photo-)chemistry of the system under consideration. In these cases, periodicity can be taken into account, since the value of the wave function and hamiltonian repeats itself after certain intervals.

### Practical application

The module is expected to provide the quantum dynamics community with a more efficient way of treating large systems whose excited state driving forces involve periodic coordinates. When used on precomputed potentials (in G-MCTDH and vMCG), the model can improve the convergence since smaller grid sizes are needed. Used on-the-fly, it reduces considerably the amount of electronic structure computations needed compared to cartesian coordinates, since conformations that seemed far in the spanned space may be closer after applying a periodic transformation.

### Source code

Currently PerGauss resides within the Quantics software package available upon request through gitlab. For more information see the PerGauss documentation here.