# CCSD convergence

7 months 1 week ago - 7 months 1 week ago #636
by negoty

*CCSD convergence*was created by

*negoty*

Dear users and developers,

I'm very new to using MRCC and already think it's quite cool. I came across an issue and couldn't find anything about it in the manual so far.

I am currently trying to compute CCSD(T) of (closed shell) [Fe(H

Thanks in advance!

Input:

Output snippet:

I'm very new to using MRCC and already think it's quite cool. I came across an issue and couldn't find anything about it in the manual so far.

I am currently trying to compute CCSD(T) of (closed shell) [Fe(H

_{2}O)_{6}]^{2+}and am encountering convergence issues in both the SCF and CCSD iterations. I was able to solve the SCF issues. Are there ways to speed up or help CCSD convergence along aside from increasing the number of iterations? It is increasingly slow and I am unsure why.Thanks in advance!

Input:

```
# CCSD(T)
calc=CCSD(T)
mem=100000MB
mult=1
charge=2
scfmaxit=50
scfext=15
scfdamp=0.7
scflshift=0.25
scfiguess=ao
#qscf=AugHessL
basis=atomtype
Fe:cc-pwCVTZ
O:cc-pCVDZ
H:cc-pVDZ
core=corr
unit=angs
geom=xyz
19
Fe 0.000000000 0.000000000 0.000000000
O 0.000000000 0.000000000 1.989595000
H -0.789066000 0.002150000 2.555337000
H 0.789066000 -0.002150000 2.555337000
O -0.036065000 1.989327000 0.000000000
H -0.007113000 2.554548000 0.788946000
H -0.007113000 2.554548000 -0.788946000
O -1.989619000 -0.009611000 0.000000000
H -2.547156000 0.785416000 0.000000000
H -2.563405000 -0.792705000 0.000000000
O 0.000000000 0.000000000 -1.989595000
H 0.789066000 -0.002150000 -2.555337000
H -0.789066000 0.002150000 -2.555337000
O 0.036065000 -1.989327000 0.000000000
H 0.007113000 -2.554548000 -0.788946000
H 0.007113000 -2.554548000 0.788946000
O 1.989619000 0.009611000 0.000000000
H 2.547156000 -0.785416000 0.000000000
H 2.563405000 0.792705000 0.000000000
```

Output snippet:

```
Starting CCSD iterations...
Residual norm Energy
Iteration 1 4.274088814384 -1719.550464222440
Iteration 2 6.484609056622 -1719.842522232327
Iteration 3 2.193209171674 -1720.033668644362
Iteration 4 1.980510710749 -1720.209594806344
Iteration 5 2.002652643973 -1720.345925538991
Iteration 6 1.239898116240 -1720.413527453012
Iteration 7 0.726006191715 -1720.461212885563
Iteration 8 0.565702744661 -1720.507344600944
Iteration 9 0.484284282073 -1720.543777687296
Iteration 10 0.371546782703 -1720.565270711646
Iteration 11 0.287244841891 -1720.578718665980
Iteration 12 0.245685775346 -1720.588653602694
Iteration 13 0.196006045394 -1720.595537426124
Iteration 14 0.147666442792 -1720.599459234976
Iteration 15 0.116515619156 -1720.601980166463
Iteration 16 0.101014030733 -1720.603678087391
Iteration 17 0.083979696661 -1720.604643284147
Iteration 18 0.066517805798 -1720.605170933782
Iteration 19 0.052176645444 -1720.605607293077
Iteration 20 0.044106502967 -1720.606030554397
Iteration 21 0.037077658969 -1720.606359059045
Iteration 22 0.030265135499 -1720.606527785491
Iteration 23 0.024271261951 -1720.606576605529
Iteration 24 0.020425249901 -1720.606588491923
Iteration 25 0.017028052329 -1720.606612637738
Iteration 26 0.013649564313 -1720.606645937079
Iteration 27 0.011383525498 -1720.606666868290
Iteration 28 0.009686951664 -1720.606677923058
Iteration 29 0.008139587728 -1720.606693398128
Iteration 30 0.006869743029 -1720.606716792576
Iteration 31 0.005807696774 -1720.606740604099
Iteration 32 0.004866826676 -1720.606754073281
Iteration 33 0.004171591320 -1720.606755069804
Iteration 34 0.003652780662 -1720.606748737499
Iteration 35 0.003078183214 -1720.606742609954
Iteration 36 0.002677137847 -1720.606744787171
Iteration 37 0.002440727451 -1720.606756613783
Iteration 38 0.002151469817 -1720.606768501056
Iteration 39 0.001986561378 -1720.606773601665
Iteration 40 0.001918707880 -1720.606774806203
Iteration 41 0.001929114539 -1720.606777488375
Iteration 42 0.001784663811 -1720.606782974503
Iteration 43 0.001730907738 -1720.606791820029
```

Last edit: 7 months 1 week ago by negoty.

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7 months 1 week ago #637
by kallay

Best regards,

Mihaly Kallay

Replied by

*kallay*on topic*CCSD convergence*
You should increase the maximum number of CC iterations, see keyword ccmaxit in the manual. But please be careful as your systems seems to be multireference.

Best regards,

Mihaly Kallay

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7 months 1 week ago #639
by kallay

Best regards,

Mihaly Kallay

Replied by

*kallay*on topic*CCSD convergence*
I have taken a closer look at the problem. The occupation changes over the iterations. You should manually set the occupation:

occ=15,6,7,14

or turn off symmetry:

symm=off

occ=15,6,7,14

or turn off symmetry:

symm=off

Best regards,

Mihaly Kallay

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7 months 5 days ago #640
by nagypeter

Replied by

*nagypeter*on topic*CCSD convergence*
Hello!

I have done some tests too.

I used the same input file and the cc-pwCVTZ basis set from the EMSL database.

As suggested by Mihály, increasing the ccmaxit variable will help, the CCSD iteration converged for me in 79 iterations.

Reference energy [au]: -1718.212045286579

CCSD correlation energy [au]: -2.394753844090

CCSD(T) correlation energy [au]: -2.418205473490

At this point there seems to be an issue with the MP2 guess of the CCSD iteration, but the energies are correct (except for the MP2). That is why it needs abnormally large number of iterations. We will try to look at it.

I think you should check your structure. The Fe-O bond length in your input matches closely the [Fe(H

Is there any other case where this situation occurred for you?

Thank you,

Peter

I have done some tests too.

I used the same input file and the cc-pwCVTZ basis set from the EMSL database.

As suggested by Mihály, increasing the ccmaxit variable will help, the CCSD iteration converged for me in 79 iterations.

Reference energy [au]: -1718.212045286579

CCSD correlation energy [au]: -2.394753844090

CCSD(T) correlation energy [au]: -2.418205473490

At this point there seems to be an issue with the MP2 guess of the CCSD iteration, but the energies are correct (except for the MP2). That is why it needs abnormally large number of iterations. We will try to look at it.

I think you should check your structure. The Fe-O bond length in your input matches closely the [Fe(H

_{2}O)_{6}]^{3+}value and it is quite far from the [Fe(H_{2}O)_{6}]^{2+}equilibrium distance. I have tried a longer Fe-O bond length value, but the slow convergence remained.Is there any other case where this situation occurred for you?

Thank you,

Peter

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7 months 5 days ago - 7 months 5 days ago #641
by nagypeter

Replied by

*nagypeter*on topic*CCSD convergence*
With the help of Mihály we have found the solution.

You should set only the following in your input:

calc=CCSD(T)

mem=7000MB

mult=1

charge=2

occ=15,6,7,14

basis=atomtype

Fe:cc-pwCVTZ

O:cc-pCVDZ

H:cc-pVDZ

core=corr

unit=angs

geom=xyz

...

The slow convergence in the HF was caused by the frequent changes in occupation. This can be fixed by setting occ, or turning off the spacial symmetry via symm=off, as suggested before. There is no need to change any other setting of the scf program, it converges in 22 steps nicely.

If you also set verbosity=3, you can follow the above in more details.

The problem in the MP2 and CCSD was originated from a bug that emerges only if damping is set in the input for the scf. So you should avoid damping, the bug will be fixed in the next release.

With this input the CCSD converges in 18 steps to the final result of:

Reference energy [au]: -1718.212045286700

MP2 energy [au]: -1720.570336329711

CCSD correlation energy [au]: -2.394753972428

CCSD(T) correlation energy [au]: -2.444087223663

I hope this solves all of your problems.

Thanks for pointing out this issue and for using our program,

Peter

You should set only the following in your input:

calc=CCSD(T)

mem=7000MB

mult=1

charge=2

occ=15,6,7,14

basis=atomtype

Fe:cc-pwCVTZ

O:cc-pCVDZ

H:cc-pVDZ

core=corr

unit=angs

geom=xyz

...

The slow convergence in the HF was caused by the frequent changes in occupation. This can be fixed by setting occ, or turning off the spacial symmetry via symm=off, as suggested before. There is no need to change any other setting of the scf program, it converges in 22 steps nicely.

If you also set verbosity=3, you can follow the above in more details.

The problem in the MP2 and CCSD was originated from a bug that emerges only if damping is set in the input for the scf. So you should avoid damping, the bug will be fixed in the next release.

With this input the CCSD converges in 18 steps to the final result of:

Reference energy [au]: -1718.212045286700

MP2 energy [au]: -1720.570336329711

CCSD correlation energy [au]: -2.394753972428

CCSD(T) correlation energy [au]: -2.444087223663

I hope this solves all of your problems.

Thanks for pointing out this issue and for using our program,

Peter

Last edit: 7 months 5 days ago by nagypeter.

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7 months 3 days ago - 7 months 3 days ago #642
by negoty

Replied by

*negoty*on topic*CCSD convergence*
Hi and thanks a ton for your help! I'm currently on vacation but it seems that all my problems have been solved in my absence (which is awesome). I'll see to it as soon as I'm back.

Concerning the occupation numbers: I noticed that they change during the SCF but I wasn't sure how to determine the correct numbers. I'm not used to utilising symmetry so is there a way to know the numbers I should aim for?

Thanks again!

Concerning the occupation numbers: I noticed that they change during the SCF but I wasn't sure how to determine the correct numbers. I'm not used to utilising symmetry so is there a way to know the numbers I should aim for?

Thanks again!

Last edit: 7 months 3 days ago by negoty.

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