Nature 221, 947 (1969);Although the indirect spin-spin interaction between nuclei in molecules has been utilized extensively for qualitative understanding of electronic structures in molecules, its quantitative calculation even for the simple molecule HD has proved to be a formidable task, with the current situation far from conclusive. 1 " 8 The various theoretical approaches utilized so far can be divided broadly into two categories. In the first category is the conventional second-order perturbation as first proposed by Ramsey and Purcell. 1 ' 2 The difficulty with such an approach is that one requires a knowledge of the complete set of ground and excited states of the molecule, which is not usually available. 3 To obviate the knowledge of excited states, variation-perturbation procedures have been used by a number of authors. 4 " 8 In one class of such calculations, a diagonal-type perturbation procedure 4 * 5 was used, the second-order energy due to the hyperfine field of one nucleus being minimized to obtain the first-order perturbed wave function of the molecule. This function was then utilized to calculate the cross term in the second-order energy involving the other nucleus to obtain J HD . The difficulty with this procedure was that the second-order nuclear self-coupling W. M. Neupert and M" Swartz, Astrophys. energy is infinite in nonrelativistic theory, no real minimum thus being attainable in a variational approach. A second class of variation perturbation calculation has attempted to extremize the cross terms in the second-order energy proportional to J HD using variational functions which describe the first-order perturbation due to both nuclei. 6 " 9 The difficulty with this procedure is that the cross term due to two perturbations has by itself no minimum and one does in fact get oscillatory behavior as the number of parameters increased. 9 In this paper, we have revived the perturbation approach in a form that meets the major difficulty, namely, a knowledge of a complete set of states for the molecule. This is accomplished by using the linked-cluster many-body perturbation theory (LCMBPT), 10 where a neighboring Hamiltonian 3C 0 , for which the complete set of states can be obtained exactly, is used as the starting point for a perturbation treatment of A3C = 3C-3CQ. In our work here, a multiple perturbation approach is used in conjunction with the LCMBPT, using the sum of AJC and the two hyperfine interaction Hamiltonians 3C H ' and 3C D ' associated with the two nuclei. For the speed of convergence of the perturbation approach, it is nec-The linked-cluster many-body perturbation approach has been applied to the study of indirect nuclear spin-spin coupling constant J HD in HD molecule. The complete set of states used were the bound and continuum states of H 2 + molecular ion with the internuelear separation appropriate to H 2 molecule. Our calculated value of J HD through the Fermi contact interaction mechanism is +42.57 Hz in good agreement with the most recent experimental value of 442.7±0.7 ...