EOM-CCSD problem, all excitation energies go to zero

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Hi,

I'm trying to run a EOM-CCSD calculation on a small metal oxide cluster using the input file quoted below. The EOM-CCSD part initially appears to converge fine to finite positive excitation energies but then suddenly all excitation energies go to ~ 0:

Iteration  55 using  191 trial vectors
0.0000000185485 0.1308371004755 3.56026
0.0000000217031 0.1514621903177 4.12150
0.0000000502665 0.1557777882543 4.23893
0.0000001651624 0.1592055635125 4.33221 39.6 95.0

Iteration  56 using  192 trial vectors
0.0000000185310 0.1308371004725 3.56026
0.0000000217483 0.1514621903270 4.12150
0.0000000502447 0.1557777882534 4.23893
0.0000002061116 0.1592055636822 4.33221 37.9 96.4

Iteration  57 using  193 trial vectors
0.0000000206880 0.1308371004792 3.56026
0.0972397023418 0.1512362019195 4.11535
0.0000010404410 0.1514621880161 4.12150
0.0000000509268 0.1557777882525 4.23893 41.7 103.5

Iteration  58 using  195 trial vectors
0.2293048302535 0.0497522482240 1.35383
0.0000000185506 0.1308371004717 3.56026
0.0000000131591 0.1514621903330 4.12150
0.0000000464140 0.1557777882910 4.23893 41.7 101.0

Iteration  59 using  196 trial vectors
0.1291815713422 0.0111206134933 0.30261
0.0000000184843 0.1308371004743 3.56026
0.0000000129946 0.1514621903290 4.12150
0.0000000464518 0.1557777882908 4.23893 39.6 98.6

Iteration  60 using  197 trial vectors
0.0547753241375 0.0017883588845 0.04866
0.0000000184471 0.1308371004725 3.56026
0.0000000128898 0.1514621903422 4.12150
0.0000000464864 0.1557777882977 4.23893 40.2 100.0

Iteration  61 using  198 trial vectors
0.0222281569063 0.0001943932224 0.00529
0.0000000184456 0.1308371004724 3.56026
0.0000000128447 0.1514621903391 4.12150
0.0000000464965 0.1557777882961 4.23893 40.2 100.4

Some steps later they all become essentially zero:

Iteration 100 using  309 trial vectors
0.0000321101923 -0.0000005642338 -0.00002
0.0000157935150 -0.0000001284763 0.00000
0.0000000312833 0.0000000000104 0.00000
0.0001271946947 0.0000012507538 0.00003 81.6 186.7

Could someone please tell me what the problem might be here? This is the input I used:

START tio2_2_clubeomcc
MEMORY 1800 MB
GEOMETRY
Ti 1.5800350 0.0816697 0.0000000
O 0.6579338 -1.4182969 0.0000000
O 0.5150702 0.7437215 -1.2499304
O 0.5150702 0.7437215 1.2499304
Ti -0.8204234 -0.0468890 0.0000000
O -2.4476859 -0.1039269 0.0000000
END
BASIS
* library cc-pvdz
END
SCF
SINGLET
RHF
END
TCE
CCSD
FREEZE CORE ATOMIC
NROOTS 4
END
TASK TCE ENERGY

Thanks in advance,

Martijn

  • Karol Forum:Admin, Forum:Mod, NWChemDeveloper, bureaucrat, sysop
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Dear Martijn
There is no need to converge the results to the 10^-8.
Using 10^-5 is sufficient to have converged excitation energies.
The instability you are pointing to (which is associated with the fact
that the solvers is trying to identify 0 as the eigenvalue - with
"singular"-type eigenvector) sometimes occurs in the EOMCC solvers,
especially, once you are using too tight conv. criteria.
Best,
Karol

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EOM-CCSD problem, all excitation energies go to zero - Solved
Hi Karol,

Thanks! Reducing the tolerances via thresh 1.0d-5 did the trick.

Best,

Martijn


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