Hi all,

This is my first post here, so I think it is well-written and all, but apologies in advance if that is not the case. I have also been reading a bit the manual and forum and not found anyone encountering this problem before? So, here it goes.

I am running some small CCSD(T) calculations with an aug-cc-pVDZ basis set to get a correction for some interaction energies by taking the difference of the CCSD(T) and MP2 correlation energies with some “small” basis set and then add that to my MP2/CBS energies. I think that is fairly standard, but just to explain what I am trying to do.

I encountered the following issue: my MP2 energies from the MP2 calculation do not match the MP2 energies from the CCSD(T) calculation. This led me to look into the output a bit more carefully, and I have noticed that it prints two different sets of energies for the CC calculation: an “SCF energy” and a “Reference energy” that are slightly different. (or in some cases not so slightly…)

Just to showcase the “problem” in the forum, I have run some calculations on the water dimer.

Psi4 version:

```
Psi4: An Open-Source Ab Initio Electronic Structure Package
Psi4 1.7 release
Git: Rev {} zzzzzzz
```

Input file:

```
memory 200000 mb
molecule dimer {
0 1
O 1.327706 0.106852 0.000000
H 1.612645 -0.413154 0.767232
H 1.612645 -0.413154 -0.767232
--
0 1
O -1.550676 -0.120030 -0.000000
H -0.587091 0.053367 -0.000000
H -1.954502 0.759303 -0.000000
units angstrom
no_reorient
}
set globals {
basis aug-cc-pvdz
guess sad
scf_type df
freeze_core true
reference rhf
}
# define dimer
dimer = dimer.extract_subsets([1,2])
psi4.IO.set_default_namespace('dimer')
Edim = energy('method',molecule=dimer)
```

Where ‘method’ is either mp2 or ccsd(t).

The input looks weird, but my bigger picture input basically does the calculation of both the dimer and the monomers, so that is why I use this perhaps too convoluted input structure.

For my ‘mp2’ calculations I obtain the following energies:

```
@DF-RHF Final Energy: -152.08641653209526
[more output]
-----------------------------------------------------------
==================> DF-MP2 Energies <====================
-----------------------------------------------------------
Reference Energy = -152.0864165320952566 [Eh]
Singles Energy = -0.0000000000000000 [Eh]
Same-Spin Energy = -0.1136800413218990 [Eh]
Opposite-Spin Energy = -0.3298954078288628 [Eh]
Correlation Energy = -0.4435754491507619 [Eh]
Total Energy = -152.5299919812460132 [Eh]
-----------------------------------------------------------
```

So the SCF energy and the “Reference Energy” printed at the end are the same. Great!

When I run the ‘ccsd(t)’ calculations I obtain the following:

```
@DF-RHF Final Energy: -152.08641653209514
[more output]
**************************
* *
* CCENERGY *
* *
**************************
Nuclear Rep. energy (wfn) = 36.636568219379491
SCF energy (wfn) = -152.086416532095143
Reference energy (file100) = -152.086456453401240
[more output]
SCF energy (wfn) = -152.086416532095143
Reference energy (file100) = -152.086456453401240
Opposite-spin MP2 correlation energy = -0.330005655604294
Same-spin MP2 correlation energy = -0.113586726584186
Singles MP2 correlation energy = -0.000000000000000
MP2 correlation energy = -0.443592382188480
* MP2 total energy = -152.530048835589724
Opposite-spin CCSD correlation energy = -0.358014535663327
Same-spin CCSD correlation energy = -0.100495579966179
Singles CCSD correlation energy = 0.000000000000000
CCSD correlation energy = -0.458510115629506
* CCSD total energy = -152.544966569030748
```

So, the SCF energy (wfn) is the same one as I obtained before. However, the “Reference energy (file100)” is different. In this case it is not by a lot, by in my other calculations it seems a bit more important. The “MP2 total energy” printed in the CCSD(T) calculation is also different from the one obtained in the MP2 calculation.

I am not sure what is the problem, and I assume it has to do with the “file100” and the fact that that energy/wavefunction is taken as a reference for the MP2/CC calculations in the second case.

I also thought that maybe it is somehow related to the density fitting, but I am not sure, so I wanted to check.

Any help is greatly appreciated!!

Thanks in advance,

Carlos