Efficient evaluation of accuracy of molecular quantum dynamics using dephasing representation

Ab initio methods for the electronic structure of molecules have reached a satisfactory accuracy for calculations of static properties but remain too expensive for quantum dynamics calculations. We propose an efficient semiclassical method for evaluating the accuracy of a lower level quantum dynamics, as compared to a higher level quantum dynamics, without having to perform any quantum dynamics. The method is based on the dephasing representation of quantum fidelity and its feasibility is demonstrated on the photodissociation dynamics of CO2. Our accuracy test can be easily implemented in existing molecular dynamics codes, thus offering wide applicability.