Different methods for the two-nucleon t-matrix in the operator form

We compare three methods to calculate the nucleon–nucleon t-matrix based on the three-dimensional formulation of Golak et al. (Phys Rev C 81:034006, 2010). In the first place we solve a system of complex linear inhomogeneous equations directly for the t-matrix. Our second method is based on iterations and a variant of the Lanczos algorithm. In the third case we obtain the t-matrix in two steps, solving a system of real linear equations for the k-matrix expansion coefficients and then solving an on-shell equation, which connects the scalar coefficients of the k- and t-matrices. A very good agreement among the three methods is demonstrated for selected nucleon–nucleon scattering observables using a chiral next-to-next-to-leading-order neutron–proton potential. We also apply our three-dimensional framework to the demanding problem of proton–proton scattering, using a corresponding version of the nucleon–nucleon potential and supplementing it with the (screened) Coulomb force, taken also in the three-dimensional form. We show converged results for two different screening functions and find a very good agreement with other methods dealing with proton–proton scattering.