III. Methodological developments and modelling tools
a. Force-fields
Depending on the system of interest and the conditions of the study, the description of the various interactions among the species, so called force fields (FF) may need to be developed, i.e. finding the most suitable analytical formula to represent the various type of interactions. In our approaches the analytical formula as well as the derived parameters are adjusted on highly accurate QM structures and energies of reasonable size (dimer interaction, clusters). For our systems of interest, covering a wide range of context from atmospheric, astrophysics, to nuclear fuel recycling, the group has developed FF for neutral and charged species accounting for polarization effects (L. Hormain, M. Monnerville, C. Toubin, D. Duflot, B. Pouilly, S. Briquez, M. I. Bernal-Uruchurtu, and R. Hernández-Lamoneda, J. Chem. Phys. 142, 144310 (2015) [DOI:10.1063/1.4917028], F. Réal, A. S. P. Gomes, Y. O. Guerrero Martínez, T. Ayed, N. Galland, M. Masella, and V. Vallet, J. Chem. Phys. 144, 124513 (2016) [DOI:10.1063/1.4944613]) and many-body corrections when required (F. Réal, M. Trumm, B. Schim- melpfennig, M. Masella, and V. Vallet, J. Comput. Chem. 34, 707 (2013)[doi:10.1002/jcc.23184]).
a. Relativistic correlated methods
We develop electronic structure approaches that describe relativistic as well as electron correlation effects for systems across the periodic table. One family of methods is based on solving the Dirac equation for the electrons (four-component and two-component methods) so that spin-orbit coupling is treated at the mean-field level, while electron correlation for ground and excited states is introduced with coupled cluster (CC, EOM-CC) (A. Shee, T. Saue, L. Visscher, and A. S. P. Gomes, J. Chem. Phys. 147, 174113 (2018); doi:10.1063/1.5053846). These developments are made on the Dirac code (http://diracprogram.org).
Spinor spin magnetization of valence spinors halogen oxide anions (ClO-, IO- and TsO-). The low-lying electronic states of the halogen oxide radicals arise from ionizations of these orbitals (doi:10.1063/1.5053846). Relativistic effect such as spin-orbit coupling are very important for heavy elements.
A second family of methods treats spin-orbit coupling via a spin-orbit CI (SOCI) [cite EPCISO] approach on the basis of many-electron wavefunctions obtained with approximate relativistic Hamiltonians, describing only scalar relativistic effects (one-component methods). These developments are made on the EPCISO code.
b. Quantum embedding
Applying relativistic CC methods to get energies and spectroscopic properties for complex systems is very costly. We develop an embedding method – frozen density embedding (FDE) (A. S. P. Gomes and C. R. Jacob, Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 108, 222 (2012) [doi:10.1039/C2PC90007F], A. S. P. Gomes, C. R. Jacob, and L. Visscher, Phys. Chem. Chem. Phys. 10, 5353 (2008). [doi:10.1039/B805739G], S. Höfener, A. S. P. Gomes, and L. Visscher, J. Chem. Phys. 136, 044104 (2012); [doi:10.1063/1.3675845], S. Höfener, A. S. P. Gomes, and L. Visscher, J. Chem. Phys. 139, 104106 (2013) [doi:10.1063/1.4820488], M. Olejniczak, R. Bast, and A. S. P. Gomes, Phys. Chem. Chem. Phys. 19, 8400 (2017) [doi:10.1039/C6CP08561J]) - that let us treat a small part of the system with relativistic CC and the rest with more approximate methods such as DFT, giving us a fully quantum mechanical description of the whole system at an affordable computational cost. These developments are done on the Dirac (http://diracprogram.org), Dalton (http://daltonprogram.org) and ADF(http://www.scm.com) codes, as well as on the PyADF scripting framework (C. R. Jacob, S. M. Beyhan, R. E. Bulo, A. S. P. Gomes, A. W. Götz, K. Kiewisch, J. Sikkema, and L. Visscher, J. Comput. Chem. 32, 2328 (2011) [doi:10.1002/jcc.21810]), which let us combine different codes.
c. Topographic characterization of potential energy surfaces
We have recently generalized the Transition State Search Using Chemical Dynamics methodology (TSSCDS) (Emilio Martínez-Núñez, J. Comp. Chem. 36, 222 (2015); DOI: 10.1002/jcc.23790] to be able to automatically determine stationary points on intermolecular potential energy surfaces, thus expanding the previous limits to non-covalent molecular systems. The so-called vdW-TSSCDS method (http://forge.cesga.es/wiki/g/tsscds/HomePage) can be applied to both reactive and non-reactive systems [S. Kopec, E. Martínez-Núñez, J. Soto, D. Peláez, (submitted)].
We have simulated the photoelectron spectra of halogens in water droplets with CC-in-DFT embedding (Y. Bouchafra, A. Shee, F. Réal, V. Vallet, and A. S. P. Gomes, Phys. Rev. Lett. 121, 266001 (2018) [doi : 10.1103/PhysRevLett.121.266001]) over structures from classical MD simulations with polarizable force fields (F. Réal, A. S. P. Gomes, Y. O. Guerrero Martínez, T. Ayed, N. Galland, M. Masella, and V. Vallet, J. Chem. Phys. 144, 124513 (2016) [doi:10.1063/1.4944613])
d. Vibrational analysis
NORMCO and VIBMOL-GUI are programs that compute and analyses the normal modes of vibration of molecules in the harmonic approximation, to perform 1) Potential Energy Distribution (PED) , mostly used in the case of molecules; this requires the definition of non-redundant internal coordinates; 2) the Kinetic Energy Distribution (KED), mostly used in solid state physics. Both programs can be obtained by personal request to Jean-Pierre Flament (mailto:jean-pierre.flamentuniv-lille.fr).