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Least Empirical DFT Calculations for Diatomic Dissociations 
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#1
Jan3013, 10:59 AM

P: 81

A somewhat strange request. I have come across this paper:
http://jcp.aip.org/resource/1/jcpsa6/v98/i7/p5648_s1 It lists hybrid DFT results for dissociation calculations. I am looking for DFT calculations that are not as phenomenological, but can't seem to find any in the literature. Are there DFT calculations out there with less empirical functionals/less fitting to experiment? Or should I just fire up my own DFT calculations with, say Turbomole to acquire "dumb" DFT results? 


#2
Jan3113, 06:45 AM

P: 390

DFT calculations are by default ab initio. However, it is impossible to know exact electronexchange function(Thus DFT can not be entirely without empirical functions). Many approximation exists and you have to find the one that suits you best. Usually the latest ones are more accurate. e.g. LDA, LSDA, GGA etc.



#3
Jan3113, 06:52 AM

P: 81

Thanks for the info,
I think I remember the LDA approximation. Would I be right in assuming all of these approximations more or less assume some level of uniformity in electron distribution, and would therefore all be unsuitable for diatomic molecules? * By unsuitable, I mean poor compared to, say CCSDT, when it comes to dissociation. 


#4
Jan3113, 08:05 AM

Sci Advisor
P: 3,628

Least Empirical DFT Calculations for Diatomic Dissociations
Yes, DFT won't describe well e.g. van der Waals forces between the separating atoms.



#5
Jan3113, 05:13 PM

P: 435

There are some functionals which are truely first principles (e.g., LDA, PBE, TPSS). Other functionals are more empirical, or not really functionals at all (e.g., LYP itself, B3LYP, the full Minnesota set, everything containing HartreeFock exchange). Could we ask what you are looking for? I guess if you actually intent to calculate diatomic dissociations, CCSD(T) or higher coupledcluster methods (for the aroundequilibrium region and the dissociated atoms) or MRCI (for the dissociation process itself or for excited states) would be more suitable than any DFT, empirical or not.



#6
Feb413, 08:55 AM

P: 81

It's literally a couple of lines I'm looking for, regarding a report I'm writing. The trouble is every paper I come across simply says "It is well known that LDA et al are poor for dissociation", without any explicit reference to a study (presumably because the poor quality of LDA/GGA for such problems is textbook at this stage, like F=ma). What I am looking for specifically is a set of LDA/GGA calculations for diatomic dissociation (I.e. C2 N2 O2 etc.). I could do them myself with TURBOMOLE, but I guess that's not kosher if they have been previously published. 


#7
Feb413, 09:07 AM

P: 81



#8
Feb413, 01:07 PM

P: 435

A semilocal functional will thus never be able to dissociate a real molecule, simply because the process requires more than one determinant for a qualitatively correct description (e.g., to dissociate singlet H2 in a continuous, nonsymmetry broken process process, the dissociated end result must be two entangled H atoms which are coupled in an openshell singlet form, and this requires two determinants). In short: This problem is inherited from HartreeFock, and KohnShamDFT is not different enough from it to make a difference (at least not with "functionals" which do not involve actually calculating a correlated wave function in order to determine the "functional" values). 


#9
Mar213, 03:44 PM

P: 81

Me again with another silly question (I just can't seem to get the hang of Web of Knowledge)
The last set I need are dissociation calculations using Moller Plesset (Specifically MP2). I realise these also have problems associated with single reference methods (for reasons cgk outlined previously). These calculations seem to be even more elusive. 


#10
Mar413, 05:05 AM

P: 435

[1] Helgaker, Joergensen, Olsen  Molecular electronic structure theory Note, however, that there are still some dissociationrelated things that can be done with MP2 (or other singlereference methods): You can calculate  the entire potential energy surface for weakly bound complexes (i.e., systems involving only noncovalent bonding),  the potential energy surface around the equilibrium (i.e., where the molecule is still singlereferency) for covalently bonded molecules  the individual atoms (They are usually fine with highspin references). With the equilibrium PES and the individual atoms, you can calculate dissociation energies/atomization energies. But not the dissociation process itself. If you are interested in atomization energies (i.e., just the end results of the dissociation process), these often are used in the benchmarking of electronic structure methods. For example, in http://dx.doi.org/10.1063/1.2889388 , atomization energies are used to test explicitly correlated openshell MP2. But if you just look for papers citing the G2/97 paper or similar things, you will find many of them. 


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