  We explore the possibility of modifying the multiplicity of the basic cluster
in the entropy functional used in the cluster variation method so that
truncation errors owing to finite size of the basic cluster may be corrected.
The numerical value of this multiplicity is found by requiring the modified CVM
entropy functional (M-CVM) in the tetrahedron approximation for the
body-centered cubic structure to yield the exact critical temperatures for
ordering and phase separation. We demonstrate that very accurate values of the
order parameter, correlation functions and the Gibbs function can be obtained
for ordered and disordered phases at arbitrary composition and temperature with
M-CVM approach by comparing the results with those for Monte Carlo simulations.
