Hi premkumar85,
For each applied voltage bias, the software calculates the electron density, etc... and writes these properties to a file with an extension "nc". For each voltage different applied voltage value, the whole scf calculation must be re-calculated since the electron density in the scattering region also changes in ATK, which makes ATK a realistic calculation environment.
However, to shorten the calculation time, you can use a previously converged simulation's results (nc file) as a starting guess for the other voltage levels (of course for the same geometry). For more information please look at the manual for restoreSelfConsistentCalculation function.
Cheers,
Serhan