Quantum Boltzmann Equation Self-Consistent-Field for the Entropic Regularization of Mean-Field Singularities
We present a Quantum Boltzmann Equation self-consistent-field (QBE-SCF) formulation for molecular electronic structure in which the one-electron reduced density matrix is propagated in an atomic orbital basis and relaxed by a Bhatnagar-Gross-Krook collision operator toward a Fermi-Dirac equilibrium defined by the instantaneous Fock matrix. At stationarity, the converged density and Fock matrices satisfy $[\mathbf{F},\mathbf{P}]=0$, the Hartree-Fock condition. While the zero-temperature equilibrium target reduces to the integer Aufbau projector, the damped collision operator ensures the steady-state density matrix is not necessarily idempotent. This kinetic relaxation affords a dual pathway to resolve mean-field singularities. For spatial degeneracies, such as H$_3$ symmetric dissociation, zero-temperature kinetic ergodicity fractionalizes the active space to recover the Generalized Valence Bond (GVB) limit. For the conical intersection in BeH$_2$ and the H$_4$ structural distortion ($D_{2h} \rightarrow D_{4h} \rightarrow D_{2h}$), finite-temperature entropic regularization recovers correlated adiabatic surfaces from a real-valued single-reference density. By maintaining stable numerical convergence across basis-set hierarchies and resolving static correlation without multi-reference wavefunctions, these results establish kinetic relaxation as a synthesis of single-reference electronic structure and quantum statistical mechanics.