  In a compressed Nuclear Matter (NM) an increasing pressure between the
nucleons starts to increase the ratio of a nucleon Fermi to average single
particle energy and in accordance with the Hugenholtz-van Hove theorem the
longitudinal Momentum Sum Rule (MSR) is broken in a Relativistic Mean Field
(RMF) approach. We propose to benefit from the concept of enthalpy in order to
show how to fulfill the MSR above a saturation density with pressure
corrections. As a result a nucleon mass can decrease with density, making the
Equation of State (EoS) softer. The course of the EoS in our modified RMF model
is close to a semi-empirical estimate and to results obtained from extensive
DBHF calculations with a Bonn A potential, which produce the EoS stiff enough
to describe neutron star properties (mass-radius constraint), especially the
most massive known neutron star. The presented model has proper saturation
properties, including good values of a compressibility and a spin-orbit term.
