computational chemistry: critical appraisal · computational chemistry: critical appraisal mario...
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Computationalchemistry:CriticalappraisalMario BarbattiA*Midex Chair Professormario.barbatti@univ‐amu.fr
Aix Marseille Université, Institut de Chimie Radicalaire
L10
L I G H T A N DM O L E C U L E S
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Ab initio and semiempirical MRCI produce divergent results
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(°)
(°)
L I G H T A N DM O L E C U L E S
4• Plasser, Crespo‐Otero, Pederzoli, Pittner, Lischka, Barbatti, JCTC 10, 1395 (2014)
Experi
mental
ADC(2)MRCIS
OM2/MRCI
TD-PBE
TD-B97
XDTD-B
3LYP
TD-PBE0
TD-CAM-B
3LYP
TD-BHLY
PTD-M
06-H
F
0
20
40
60
80
S 0 po
pula
tion
at 1
ps
(%)
C2 puckering C6 puckering H elimination Experimental682
5712
858
598
58128
2015
0
20212516
1016
TDDFT: unable to predict ps behavior
Right for different reasons
L I G H T A N DM O L E C U L E S
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2.0
2.5
3.0
3.5
4.0
4.5
B97X-D(6-311G(df,2p))
M06-HFB3LYP PBE0 B97X-D(6-31G(d))
M06-2X CAM-B3LYP
LC-BLYP( = 0.47)
LC-BLYP( = 0.29)
Ener
gy (e
V)
LC-BLYP( = 0.2)
f > 0.2
LOC(A)LOC(D)CT DACT ADDELOC
• Sen, Crespo‐Otero, Weingart, Thiel, Barbatti, JCTC 9, 533 (2013)
D
A
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• Excited‐state spectral region has high density of states• Small variation in geometry leads to a change of electronic character• No affordable method can describe all characters at the same level
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Energy
Time
SR
ia SR
ja SR
SR
i ja a SR
i ja a SR
MR
MR
Dynamic Electron Correlation
Non‐Dynamic EC
If DEC is needed everywhere and NDEC only at the intersection, why choosing CASSCF (only DEC) for dynamics?
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Up to here, dynamics may be OK.But we need better research protocolsto guarantee quality
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11• Barbatti, Crespo‐Otero, Top Curr Chem 368, 415 (2015)
Diagonal XC kernel for CT stateshas wrong behavior
Single‐determinant KSapproximation fails; self‐
interaction errors
Linear response and ALDA approximations
may fail
No problem, in principleFunctional‐dependent
predictions
DFT is, in principle, valid
Functional‐dependentpredictions
Energy
xy Linear response fails for
multiple excitations
L I G H T A N DM O L E C U L E S
1T 2T 3T 4T0.0
0.5
1.0 9T (expt)
Excited Population at 300 fs
12• Fazzi, Barbatti, Thiel, PCCP, accepted (2015)
TD‐B97XD/6‐31G*
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Computational chemistry of excited states:
• Powerful tool to understand mechanisms and interpret experiments• Due to very complex electronic densities, they demand high expertise• Used like black‐box they may lead to qualitatively wrong results
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Reaction pathways:
• They can be computed with very high computational levels• However, they are limited to few biased coordinates based on “educated guess”
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Semiclassical Dynamics:
• All degrees of freedom can be explored• It reveals features that we would otherwise miss • However, the computational level must be strongly downgraded
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The best we can do is to combine them, reaction paths and dynamics
• Start with reaction paths comparing low/high levels and different methods• Do low level‐dynamics to let the important degrees revel themselves• Move to high‐level reaction paths to check the results• If things are not consistent, go back, improve dynamics level• Repeat the steps till self‐consistence is achieved
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The Five Commandments
I. Thou shalt use dynamics only for what is really necessary
II. Test thy inputs and outputs before submitting thy real production jobs
III. Thou shalt NOT let computers running alone without checking what is going on
IV. Thou shalt NOT start dynamics without a very careful investigation of the theoretical level
V. Thou art a limited and biased being, therefore, thou shalt NOT rely on thy eyes to perform analysis
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• Use automatic approach for data input and analysis.
• Test the results with methods from different families (DFT x CC, for example).
• Improve statistics and compute statistical errors.
• Limit analysis to significant results with the error bar.
• Don’t blindly rely on previous works. Reanalyze their data.
• Be rigorous and honest. Report negative results and failures.