Theory and Simulation of Molecular Interactions

Intermolecular forces, or equivalently, interactions, are responsible for most of the physical properties of the matter at conditions of temperature and pressure that we experience in our everyday life: the behavior of the real gases, the gas-liquid phase transitions, capilarity, adsorption, etc.

The concept of intermolecular forces constitutes a powerful paradigm as, on the one hand, it allows us to explain the outcomes of molecular collisions in a broad range of conditions and the spectroscopy of molecular clusters bounded by these forces or, on, the other hand, macroscopic properties of gases and condensed phases.

Within the Born-Oppenheimer approximation, equations of motion for the electrons of the molecular system are first solved for every position of the nuclei and this process sets up a Potential Energy Surface (PES) governing the motion of the nuclei. The second step involves the solution of the dynamical problem of the nuclear motion. Several challenges are related to this topic:

  • The development of efficient methods for studying ever larger molecular systems.
  • The treatment of open-shell systems where electrons are unpaired.
  • The range of validity of the property of additivity of the intermolecular forces in molecular clusters (role of many-body forces).

Our research line focuses in building global and reliable intermolecular PESs and in the description of the nuclear motion on those PESs. We primarily follow a quantum-mechanical ab initio approach, although more approximated treatments are also used for the most complex systems. We intend a close exchange with experimental groups of the field. We aim to provide data (molecular properties, intermolecular potentials, collision rate coefficients) which are useful to other fields: astrochemistry, atmospheric, combustion or surface science etc., particularly for extreme or experimentally unaccesible conditions.

Our scientific objectives are:
a) Calculations of (weak) molecular interactions in gas phase: energy transfer in molecular collisions (rate coefficients in a broad range of temperatures), molecular structure and spectroscopy.
b) Applications to astrochemistry, atmospheric science, etc. Studies of molecular clusters: validity of pairwise additivity, structure, stability, condensation mechanisms, etc.
c) Extended systems: interactions between atoms and molecules with carbon-based molecules and surfaces (Polycyclic Aromatic Hydrocarbons, graphene, etc.) and dynamical simulations (adsorption, diffraction, etc.) Applications to surface and materials science.
To accomplish such objectives, we employ several methods

I) Obtention of global PESs from ab initio electronic structure calculations. b) Modelization of global PESs from approaches based in sounded physical concepts (polarizability).

II) Quantum-mechanical calculations of molecular collision dynamics (time-independent and time-dependent Schrödinger equation).

III) Structure and spectroscopy of molecular dimers and small clusters: geometry optimizations, bound states calculations, Difussion Monte Carlo methods, etc.