Time-dependent coupled-cluster for ultrafast spectroscopy

The ultimate reason for chemical reactivity is the electronic motion, occurring at an attosecond timescale. Until the last century, it was impossible to observe it directly, as the shortest available laser pulses had duration in the order of femtoseconds. Recent technological advances lead to sub-femtosecond laser pulses, making possible real-time observation and control of electron dynamics.My Ph.D. thesis aims to develop and implement a model for the interaction between ultrashort laser pulses and molecules. This is interesting as an extension of the theory and the computational tools available, to design experiments at laser facilities, and to predict and interpret their outcomes.The theoretical framework that we have chosen is the time-dependent coupled-cluster (TDCC) theory. We have implemented our code in the eT program, which represents the first released implementation of a TDCC method.After validating our procedures by comparison with the literature, we used our code to calculate the electronic response to a pump-probe sequence of laser pulses. We performed convergence tests of parameters on the LiH. Then, we observed and interpreted the effect of the delay between pump and probe pulses on the LiF transient absorption spectrum.We extended this implementation to a time-dependent equation-of-motion coupled-cluster (TD-EOM-CC) approach with the use of a reduced basis calculated with an asymmetric band Lanczos algorithm, and within the core-valence separation (CVS) approximation. This converged to the same spectral features as the TDCC but with much lower computational times, as we showed for LiF. We observed the limits of CVS approximation: for the LiH molecule, several peaks were not correctly retrieved. Finally, we modeled the transient absorption for the glycine molecule, which is a good candidate for experimental investigations.We also modeled the electronic impulsive stimulated Raman scattering (ISXRS) population transfer induced by an ultrashort laser pulse through the TD-EOM-CC model for Ne, CO, pyrrole, and p-aminophenol and visualized through a movie the real-time evolution of the electronic density of p-aminophenol.The significance of this work lies in the development of theoretical and computational tools to be used in attochemistry: one groundbreaking application can be the direct control of electrons, which would have a big impact on many research fields, like medicine, biology, and material science.