Quasi Diabatic Representation for Nonadiabatic Quantum Dynamics Propagations

We develop a non-adiabatic quantum dynamics propagation scheme that allows interfacing diabatic quantum dynamics approaches with routine adiabatic electronic structure calculations. This new scheme is based a quasi-diabatic (QD) representation that uses adiabatic states as diabatic states during a short-time quantum dynamics propagation. Between two consecutive short-time propagations, the electronic state basis is transformed from the old to the new QD representation. This scheme allows using diabatic quantum dynamics method with any adiabatic electronic structure methods, resolving the incompatibility challenge between the two methods due to their different representations. In addition, with the QD scheme one avoids explicitly computing the derivative coupling vectors and thus alleviate the numerical instability for quantum propagations in the cases where these vectors change rapidly in time. Using a recently developed diabatic quantum dynamics approach, Partial Linearized Density Matrix (PLDM) path-integral method as a specific exampel, we have demonstrated the accuracy of this QD scheme with a wide range of model non-adiabatic problems as well as the on-the-fly propagations with Density Functional Tight-Binding (DFTB) calculations.