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Keywords:

  • real time time-dependent models;
  • nonstationarity degree;
  • energy and dipole fluctuations;
  • laser fields;
  • electron correlation;
  • number of effectively unpaired electrons

Based on an earlier article (Eberly and Singh, Phys. Rev. D 1973, 7, 359) and related works on short-time evolution, this article proposes a many-electron formulation for the nonstationarity degree which can be assigned to quantum system at each time point. The key measure introduced, inline image, is a nonstationarity index that can be thought of as an inverse nominal lifetime at each instance of time. The index is directly computed from the time derivative of one-electron density matrix and inline image is a size-consistent quantity. In this article, the approach is developed for the time-dependent Hartree–Fock (TDHF), single-excitation (TDCIS), and time-dependent full configuration interaction (TDFCI) models. As a rule, nonstationarity effects are more pronounced in correlated electron systems, and a joint analysis of inline image and the multiconfigurational character of wave functions apparently provide a deeper insight into dynamical molecular processes. The performed calculations on small molecules in laser fields show a preference for the TDCIS model when comparing TDCIS and TDHF with the “exact” TDFCI model. © 2013 Wiley Periodicals, Inc.