Approximate energy expressions are proposed for infinite zero-temperature neutron matter and symmetric nuclear matter by taking into account noncentral forces. They are explicitly expressed as functionals of spin-isospin-dependent radial distribution functions, tensor distribution functions and spin-orbit distribution functions, and can be used conveniently in the variational method. The two-body noncentral cluster terms are fully included in these expressions, while the degree of inclusion of the three-body cluster terms related to noncentral forces is less than that of the purely central three-body terms. The Euler-Lagrange equations are derived from these energy expressions and numerically solved for neutron matter and symmetric nuclear matter. The Hamada-Johnston and AV14 potentials are used as the two-body nuclear force. The results show that the noncentral forces cause the total energy of symmetric nuclear matter to be decreased too much with a saturation density which is too high. Discussion is given as to the reason for this disagreement with experiment, and the long tails of the noncentral distribution functions obtained in the numerical calculations are suspected to be the main reason. Then, an effective theory is proposed by introducing a density-dependent modification of the noncentral part of the energy expression to suppress the long tails of the noncentral distribution functions. With a suitable choice of the value of a parameter included in the modification, the saturation point (both the energy and the density) of symmetric nuclear matter can be reproduced with the Hamada-Johnston potential. Neutron stars are studied by use of this effective theory, and reasonable results are obtained.
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