Large-Scale Calculation of Vibrational Sum Frequency Generation Spectra of Aqueous Interfaces

We present a software to calculate phase-resolved resonant vibrational sum-frequency generation (vSFG) susceptibility χ⁽²⁾(ω) of water and hydroxyls at planar interfaces, e.g., air/water or solid/liquid or (bio)membrane/liquid interfaces of aqueous solutions. The released code (i) reads several formats of molecular trajectories, both from ab initio (AIMD) and classical MD (CMD), (ii) calculates instantaneous surfaces to allow flexible interfaces, (iii) is written in FORTRAN, parallelized by OpenMP and optimized for memory usage, (iv) allows processing of systems up of tens of thousand atoms and for unlimited simulation time, and (v) includes many tunable processing parameters. The code and its documentation are available via GitHub. Flexible models of water and surface hydroxyl (if evaluated) (CMD or AIMD) must be used. The derivatives of the polarizability tensors and dipole moments with the change of O-H distance must be calculated externally by ab initio methods and provided as input data. We present the impact of various parameters of the MD simulations (simulation length, nonbonded interaction cutoff, size of the system, and thermostat relaxation time) as well as of the processing code (filter relaxation, cutoff of cross-terms) and provide representative results for air/water, charged quartz (101)/aqueous solution, and neutral α-alumina (0001)/aqueous solution interfaces. Further extensions are planned to distinguish signals from specific O-H or C-H bonds of interfacial molecules.