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Two different generalization schemes for the molecular field approximation are described. Both contain the possibility to incorporate generalized effective fields that induce correlations between the degrees of freedom at the boundary of a chosen cluster. Applications are presented for low-dimensional spin-1/2 systems. For the Ising models the critical couplings and critical exponents as well as the spontaneous magnetization curve are determined. An application of the method to the quantum Sϭ 1 2 one-dimensional Heisenberg model is presented and reliable low-temperature estimates of the specific heat are evaluated. The data are combined with the quantum transfer matrix predictions for large system sizes and this leads to the following prediction for the low-temperature specific heat: CϭAT ␣ with ␣ϭ0.35Ϯ0.07. ͓S1063-651X͑97͒08006-9͔