  New method for ab initio calculations of the properties of large size system
based on phase-amplitude functional is presented. It is shown that Schrodinger
equation for many electrons complex system including large size molecules, or
clusters and also periodic systems could be translated into functional of two
variables, attributed to many electron wavefunctions: phase and the amplitude
(i.e. square root of total electron density). The equations for the phase and
the amplitude are derived. The kinetic and Coulomb interaction energy are
expressed in function of these variables. The equations for one-electron
wavefunctions, necessary for the energy spectrum are derived using these two
variables.
