A general density expansion method based on the calculus of finite differences (Ramshaw J. D., J. Chem. Phys.,57 (1972) 2070) is revisited and reformulated. The original expression for the coefficients in the expansion is transformed into an alternative equivalent form which simplifies and facilitates their evaluation. The use of the method is illustrated by determining the virial coefficients of arbitrary order in a density expansion of the pressure in a classical monatomic gas. The resulting virial coefficients are expressed in terms of the Ursell reducible cluster integrals. The functional form of those expressions is determined by simple recursion relations.

重新讨论并重新制定了基于有限差分演算的通用密度扩展方法（Ramshaw JD，J。Chem。Phys。，57（1972）2070）。扩展中系数的原始表达式被转换为可替代的等效形式，该形式简化并便于对其求值。通过确定经典单原子气体中压力密度扩展中任意阶数的病毒系数来说明该方法的使用。最终的病毒系数以Ursell可约簇积分表示。这些表达式的功能形式由简单的递归关系确定。