Non‐Bivariational Time‐Dependent Orbital‐Quasi‐Optimized Coupled‐Cluster Methods Family for Fermion‐Mixtures Dynamics

ABSTRACT

Recently, [JCP 161, 114114 (2024)] successfully derived numerically stable and converging (to the time‐dependent complete active space self‐consistent‐field) time‐dependent orthogonal coupled cluster (CC) methods, namely, time‐dependent orbital optimized CC (TD‐ooCC) methods for fermion‐mixtures with arbitrary fermion kinds and numbers, via the time‐dependent bivariational principle and setting constraints in the ansatz. In this work, we extend the TD‐ooCC methods to eighteen non‐bivariational time‐dependent orbital‐quasi‐optimized coupled‐cluster (TD‐oqoCC‐NB) methods. In TD‐oqoCC‐NB methods, the constraints in the ansatz are also used to avoid overparameterization in the complete active space limit and eliminate the numerical instability in the finite truncation case. The equations of motion are not determined by the time‐dependent bivariational principle. Instead, all equations ensuring the convergence to the complete active space limit can be selected flexibly. The applications to electron dynamics and vibrational dynamics are also discussed. In particular, we find that all NB methods violate the Ehrenfest theorem even when they are gauge‐invariant, which distinguishes them from TD‐ooCC methods. The stationary condition of EOMs of each NB methods also provides novel CC methods for time‐independent electronic structure calculations.