Large - Site DMRG with custom configurations

[MaarHybrid]

I refered https://block2.readthedocs.io/en/latest/developer/orbital-rotation.html#example but it is too hard to understand for me.
So i want to ask several questions based on paper https://pubs.acs.org/doi/10.1021/acs.jctc.1c00957

1. How can I select orbitals for inactive (left), active, and external (right) sites?
   For example, suppose I have the following orbitals: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11].
   If I want to assign [0, 3, 5, 6] as inactive orbitals, [1, 2, 10] as external orbitals, and [4, 7, 8, 9, 11] as active orbitals, how should I write the code?

2. How can I manually select configurations for the inactive and external sites?
   For example, based on the inactive and external orbitals from question (1), if I want to choose three configurations (P_in = 3) for the inactive orbitals, such as {2+-0, ++++, 220-}, and two configurations (P_ext = 2) for the external orbitals, such as {202, --+}, how can I do this?

3. Does the large-site MPS still support a block-sparse structure? Also, does it support SU(2) symmetry?

4. Where should i modify if i don't use molecule symmetry?

Thank you

[hczhai]

The large-site DMRG is supported in the input file interface of block2, which can be easily understood and used. The keyword mrci can be used to set an uncontracted DMRG-MRCISD calculation (see https://block2.readthedocs.io/en/latest/user/keywords.html#uncontracted-dynamic-correlation). The keyword big_site can be used to activate the large-site DMRG mode (see https://block2.readthedocs.io/en/latest/user/keywords.html#calculation-types). Example input files can be found in #30.

1. You can use the script pyblock2/driver/gaopt to adjust the global ordering of the orbitals in a FCIDUMP file, and the output is a new FCIDUMP file (see https://github.com/block-hczhai/block2-preview/blob/master/pyblock2/driver/gaopt#L35-L36). You can then use the new FCIDUMP with the keyword noreorder to perform DMRG.
2. The arbitrary selection of CSFs in large-site DMRG is not supported.
3. The large-site MPS tensor uses block-sparse symmetry with SU(2) or U(1) group for spin quantum numbers.
4. You can modify the FCIDUMP file to set ORBSYM= with a list of 1s. For example, ORBSYM=1,1,1,1 if you have 4 orbitals. With the modified FCIDUMP, block2 will not use any point group symmetry.