We derive a microscopic expression for the instantaneous diagonal elements of the density matrix ${\rho }_{nn}(t)$ in the adiabatic basis for an arbitrary time-dependent process in a closed Hamiltonian system. If the initial density matrix is stationary (diagonal) then this expression contains only squares of absolute values of matrix elements of the evolution operator, which can be interpreted as transition probabilities. We then derive the microscopic expression for the heat defined as the energy generated due to transitions between instantaneous energy levels. If the initial density matrix is passive [diagonal with ${\rho }_{nn}(0)$ monotonically decreasing with energy] then the heat is non-negative in agreement with basic expectations of thermodynamics. Our findings also can be used for systematic expansion of various observables around the adiabatic limit.

• Received 16 June 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.220402