  After analyzing Dirac's equation, one can suggest that a well-known
quantum-mechanical momentum operator is associated with relativistic momentum,
rather than with non-relativistic one. Consideration of relativistic energy and
momentum expressions allows us to define the non-relativistic, relativistic and
pseudo-relativistic (present in Schr\"odinger equation) kinetic energy
operators. Consequences of kinetic energy operator's correction for spectra of
basic quantum Hamiltonians are investigated. In some cases this correction can
produce remarkable spectra modifications.
