Zhao, PY (reprint author), ACAD SINICA,INST HIGH ENERGY PHYS,BEIJING 100039,PEOPLES R CHINA.
In the variational cumulant expansion approach, we calculate the internal energy for the d-dimensional O(N) nonlinear a models on lattice. Specially, me give the results for two-dimensional O(3), O(4) and O(5) models. A comparison with Monte-Carlo (MC) data is also presented. Our formalism can given the results for O(N) nonlinear cr models (for any N and any dimensionality d) straightforwardly.