Time-dependent variational approach for boson systems

Applying to boson systems the time-dependent variational principle of Balia n and Veneroni, we derive approximate methods for calculating expectation v alues when both the measured observable and the density matrix are exponent ials of quadratic forms of boson operators. In the zero-temperature limit, the variational equations provide a consistent way to evaluate transition p robabilities. (C) 1999 Academic Press.