Dear Tian,
Too many thanks for your highly kind attention and prompt reply, my nice friend.
Unfortunately, for a while, I have no access to the Orca forum "https://orcaforum.kofo.mpg.de" even using the VPN! What is your opinion about my problem? Do you have access to this forum? If so, do you access normally or through using VPN?

I know that at the end of this month, the new version of Orca, Orca 6, would be released and, very unfortunately, I could not download it. Can you, by anyway you find appropriate, upload the download link on a virtual server (for instance) so I can download it?

Sincerely yours,
Saeed

Dear Saeed,

I am not sure, please ask developer in ORCA forum

Tian

Dear Tian,
If possible and, as always, you so kindly let me I am going to respectfully ask you a question which occupied my mind for a while.
My question is associated with the "MDCI" method in Orca. Please let me clarify my question using a simple example:

Suppose we are going to perform CBS extrapolation using "! ExtrapolateEP2(3/4,aug-cc,MP2) tightSCF noautostart miniprint nopop" command. In this way, the Orca output is quite evident and straightforward as:

SCF energy with basis aug-cc-pVTZ:                       -1.107346447
SCF energy with basis aug-cc-pVQZ:                       -1.115508581
Extrapolated CBS SCF energy :                            -1.118301550 (-0.002792969)

MP2 energy with basis aug-cc-pVTZ:                       -0.026103318
MP2 energy with basis aug-cc-pVQZ:                       -0.031553219
Extrapolated CBS correlation energy :                    -0.034637710 (-0.003084491)

CCSD(T) correlation energy with basis aug-cc-pVTZ:       -0.033545787
CCSD(T) - MP2 energy with basis aug-cc-pVTZ:             -0.007442470

Estimated CBS total energy :                             -1.160381730

There is no any questionable issue. But, please suppose we replace MP2 with DLPNO-CCSD(T) as:

"!ExtrapolateEP2(3/4,aug-cc,DLPNO-CCSD(T),tightPNO) tightSCF noautostart miniprint nopop"

Now, the Orca output becomes as follows:

SCF energy with basis aug-cc-pVTZ:                       -1.107346447
SCF energy with basis aug-cc-pVQZ:                       -1.115508581
Extrapolated CBS SCF energy :                            -1.118301550 (-0.002792969)

MDCI energy with basis aug-cc-pVTZ:                      -0.033546260
MDCI energy with basis aug-cc-pVQZ:                      -0.038338651
Extrapolated CBS correlation energy :                    -0.041051010 (-0.002712359)

CCSD(T) correlation energy with basis aug-cc-pVTZ:       -0.033546260
CCSD(T) - MDCI energy with basis aug-cc-pVTZ:            -0.000000000

Estimated CBS total energy :                             -1.159352560

The challenge is "MDCI energy with..." or "CCSD(T) - MDCI energy with..." phrase. I cannot understand what quantity is referred to by MDCI energy. What is its exact meaning and what is its difference by CCSD(T)? What does mean the difference calculated by "CCSD(T) - MDCI energy"?

I would be highly grateful if you kindly let me know the answer of these challenging questions. I read several times the Orca 5.0.3 manual but I cannot reach a straightforward solution.

Sincerely yours,
Saeed