Memory mapped MO Integrals

[shivupa]

If we are going to calculate all the MOs we should use a memory mapped file
https://github.com/mandreyel/mio

This could make restarts better as well

[shivupa]

The storage of MO 2e ints should be

• For small problems dense array
• For medium problems Sparse Hashmap
• For Large problems memory mapped file (sparse?)

This problem doesn't have to be addressed yet. Any current calculation fits in memory