So my back of the envelope math says heating Mars' core from 1400°C (estimated temperature, exact value unknown) to the melting point of iron of 1538°C would take 8.21*10^20J or 2.2 billion TWh. So 100.000 times the current worlds yearly energy consumption.

How you produce that energy is kind of irrelevant, but assuming we use a perfectly efficient hydrogen bomb that somehow converts all it's mass into energy using deuterium tritium fusion, we'd need approximately 10.000.000 tons of hydrogen. Half of that deuterium and half tritium. That sure is one Big nuke ;)