How to avoid drowning in the Dirac sea

An approximate (and simple) prescription was also invented to avoid the catastrophic drowning described above. Indeed, eq. (3.58) can be transformed without any problem to 

Since 2moc represents a huge energy when compared to the kinetic energy - V, then the first parenthesis on the right-hand side is to a good approximation 

Barysz, A.J. Sadlej, J.G. Snijders, Int. J. Quantum Chem. 65 (1997) 225 M. Barysz, J. Chem. Phys. 114 (2001) 9315 M. Barysz, A.J. Sadlej, / Mol. Struct. (Theochem) 573 (2001) 181 M. Barysz, AJ. Sadlej, /. Chem. Phys. 116 (2002) 2696. In the latter paper an exact solution to the problem was given. The two-component theory, although more appealing, both from the point of view of physics as well as computationally, implies a change in definition of the operators, e.g., the position operator is replaced by a quite complex expression. This fact, ignored in computations using two-component theories, has been analysed in the articles V. Kello, AJ. Sadlej, B.A. Hess,/. Chem. Phys. 105 (1996) 1995 M. Barysz, A.J. Sadlej, Theor. Chem. Acc. 97 (1997) 260 V. Kello, A.J. Sadlej, Int. J. Quantum Chem. 68 (1998) 159 V. KeUo, AJ. Sadlej,/ Mol. Struct. (Jheochem) 547 (2001) 35. 

It is seen that tr w is of the order of momentum mv, and for the small velocities 

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