  As the residual interaction of quantum chromodynamics in low-energy region,
the nucleon-nucleon (NN) potential can only be exactly described by the model
picture now. In the Bonn potential, one of the most well-known NN interaction
models, the nucleons interact with each other through exchanging the pion and
several heavier mesons, where the pion plays an essential role. It provides a
partial contribution of tensor force in the intermediate-range region and the
main component in the long-range region in NN potential. However, it is very
difficult to be treated in the nuclear many-body system due to its pseudovector
or pseudoscalar property. Recently, three high-precision charge-dependent Bonn
potentials were proposed with pseudovector coupling types and different
pion-nucleon coupling strengths and applied them to study the properties of
nuclear matter and neutron stars in the non-relativistic and relativistic
frameworks. Furthermore, to properly deal with the strong short-range repulsion
and tensor force of the NN potential, some new relativistic {\it ab initio}
methods have also been developed in the past decade to discuss the role of pion
and relativistic effects in nuclear matter.
