DENSITY-MATRIX FOR A TRIATOMIC LINEAR MOLECULE MODEL

We find the unitary operator for diagonalizing the Hamiltonian of a tr iatomic linear molecule. The unitary operator is expressed in a coordi nate representation that brings convenience for deriving the density m atrix rho(x(1),x(2),x(3);x(1)'x(2)'x(3)',beta). The unitary transforma tion approach is simple because the derivation can be reduced to the c alculation of two independent harmonic-oscillator density matrices and one free-particle density matrix. The average of the potential energy and the kinetic of the molecule is calculated by using the density ma trix.