X2C and other relativistic Hamiltonians in MRCC

Dear MRCC developers,

Thanks so much for your great work of MRCC package.

Now, MRCC become a more and more powerful quantum chemistry software.

However, MRCC lack of a native/internal support of relativistic Hamiltonians to study the heavy elements quantum chemistry. Like the famous Exact Two-Component (X2C) and many more others.

Basically, We can use the native MRCC to other interfaces to use the relativistic Hamiltonians, like CFOUR/MOLPRO/DIRAC interface, but these interfaces either lack of full functions available(only relativistic SCF/CC/CI?) or lack of maintenance(only very old version supported).

As far as I know, use the above interfaces, we cannot do a relativistic DFT geometry optimization(if wrong, please kindly correct me).

Exact Two-Component (X2C) is a very efficient/accurate relativistic Hamiltonian and also easy to code/implement, spin-free-X2C analytic gradient/hessian had been implemented in PYSCF. And the next release of ORCA will also support X2C.

So I would like to ask that whether there is any optunity to implement the Exact Two-Component (X2C) in MRCC internally.

Another very easy to implement but accurate relativistic Hamiltonian is the newly published AAA two-component Hamiltonian ( doi.org/10.1063/1.5082231 ), it is also worth to see.

Qinqing

Dear Qinqing,

Thank you for your kind words, we are glad that you find MRCC useful.

Presently only EPCs are available to handle relativistic systems without relying on the interfaces, X2C or other relativistic DFT geometry optimization options are indeed not available.

Unfortunately, the implementation of X2C and related methods were not among our short term goals, as our developer group has limited resources and so far there were little interest in such functionality in MRCC from the user side.
This is probably because there are a number of other programs that can perform such calculations.

Best wishes,
Peter

Dear Peter,

Thanks so much for your kindly reply.

So I would like there be a developer guide of MRCC code that someone can easily involve the contribution of MRCC code.

Another direction is Open-Source the MRCC code in GitHub or GitLab just like the famous Molcas/OpenMolcas and also the NWChem and many others, then more and more MRCC users will maybe make more and more contributions.

A lot of great codes/packages/programs have disappeared during the QC/CC history, maybe this is just the new supersedes the old, but what a pity that these wealths of all mankind disappeared!!!

Even though, in the future, the Quantum Computer coming, And also new computer languages and also new algorithms new QC packages coming with adaptation of Quantum Computer, all of today's 'old' QC codes/packages/programs retired! But "WE" have acquired brilliant and great achievement!!!

Best wishes,
QinQing

Dear Qinqing,

thank for contributing your ideas.
It would be interesting to know, but you did not share what kind of features you are interested in to implement.

Unfortunately, the current licence model of MRCC does not support your suggestions and such restructuring is not in our short term goals.

I have talked to Mihaly Kallay and he grants you an exception to modify the MRCC code for your own personal use. Your modifications cannot be distributed, but we would welcome to be informed about your future developments and consider to include those into our code.

Best wishes,
Peter

Dear Peter and Mihaly Kallay,

Thanks so much for you both's kindness.

It is really a great honor for me as an exception to modify the MRCC code.

I will follow the MRCC license faithfully.

Qinqing