After Shabbath is over may I say "hamavdil bein kodesh lechol" to enable myself to do melacha?

Answer

One should only do melacha after one has either said shmone esre of motzaei Shabbath (i.e., with "ata chonantanu") or havdala. Simply saying "hamavdil bein kodesh lechol" is inappropriate under ordinary circumstances.