Dear Researchers,
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
All is OK and understandable and, there is not 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 challenging question 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"?
In advance, too many thanks for any help.
Sincerely,
Saeed
- Badges
- Science topic
- Similar topics
- Chemistry