Electromagnetic form factors of the neutron-proton system in the formalism of multipole parametrization

General method of parametrization of matrix elements of local operators developed by A. A. Cheshkov and Yu. M. Shirokov is applied to parametrization of the matrix element of electromagnetic current between the two-nucleon states in terms of inelastic invariant formfactors. In the case of the asymptotic matrix element of the current (which corresponds to the non-interacting neutron and proton) the calculations are performed in the explicit form and the formulas are obtained for the formfactors which are the most important ones in the applications. These formfactors are algebraic functions of invariant variables and electromagnetic formfactors of physical nucleons. It is shown that the two-particle formfactors include new terms absent in the traditional approaches, already in the case of the $S$-state of relative motion of nucleons. These terms are due to relativistic rotation of the spin.