A fully polarizable implementation of the hybrid quantum mechanics/molecular mechanics approach is presented, where the classical environment is described through the AMOEBA polarizable force field. A variational formalism, offering a self-consistent relaxation of both the MM induced dipoles and the QM electronic density, is used for ground state energies and extended to electronic excitations in the framework of time-dependent density functional theory combined with a state specific response of the classical part. An application to the calculation of the solvatochromism of the pyridinium N-phenolate betaine dye used to define the solvent ET(30) scale is presented. The results show that the QM/AMOEBA model not only properly describes specific and bulk effects in the ground state but it also correctly responds to the large change in the solute electronic charge distribution upon excitation.

We present the implementation of a Born-Oppenheimer (BO) hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) strategy using density functional theory (DFT) and the polarizable AMOEBA force field. This approach couples the Gaussian and Tinker suite of programs through a variational formalism allowing for a full self-consistent relaxation of both the AMOEBA induced dipoles and the DFT electron density at each MD step. As the DFT SCF cycles are the limiting factor in terms of computational efforts and MD stability, we focus on the latter aspect and compare the time-reversible BO (TR-BO) and the extended BO Lagrangian approaches (XL-BO) to the MD propagation. The XL-BO approach allows for stable, energy-conserving trajectories offering various perspectives for hybrid simulations using polarizable force fields.

We present a computational strategy to simulate the absorption lineshape of a molecule embedded in a complex environment by using a polarizable QM/MM approach. This strategy is presented in two alternative formulations, one based on a molecular dynamics simulation of the structural fluctuations of the system and the other using normal modes and harmonic frequencies calculated on optimized geometries. The comparison for the case of a chromophore within a strongly inhomogeneous and structured environment, namely the intercalation pocket of DNA, shows that the MD-based approach is able to reproduce the experimental spectral bandshape. In contrast, the static approach overestimates the vibronic coupling, resulting in a much broader band.

Hybrid DFT(Gaussian)/AMOEBA(Tinker/Tinker-HP) polarizable molecular dynamics including the QM/MM mutual polarization on large complex systems. Example of the thiazole orange dye buried in a DNA double helix, embedded in a sphere of water (16 500 atoms).