As an important step in reducing computational cost in ab initio molecular dynamics presented in the previous post, we let RealQM compute atomisation energies E of X2 valence bond molecules with the X atom represented by 1 or 2 valence electrons surrounding a +1 or +2 kernel of certain radius R for a range of X and R and compare with experimental observations:
For +2 we get (code):
- R = 0 E = 0.38 compare with postulated E<0 for He2 and discussion below
- R = 0.5 E = 0.29 compare with 0.23 for C2
- R = 1 E= 0.19 compare with 0.18 for O2
- R=1.2 E= 0.14
- R=1.5 E=0.029 compare with 0.004 for Be2
We see increasing E with decreasing R as the full X kernel charge increases. Comparing with observations we can fit a proper R to X to be used in valence bond computations of molecules including X.
We use these values to compute atomisation energies for
with fair agreement with observation. Recall that HeH does not form from He+H since He+H dissociates into He+ and H. An upcoming post will investigate if HeH can form directly from He and H. So even if HeH has lower energy than He and H separate, it does not guarantee that it will form, since a physical path to lower energy is required.
For +1 we get (code):
- R = 0 E = 0.17 compare with 0.17 for H2
- R = 0.5 E = 0.09 compare with 0.11 for B2
- R = 1 E = 0.05 compare with 0.04 for Li2
- R = 1.5 E = 0.002 compare with 0.003 for Na2
We see the same pattern of increasing E with decreasing R with H2 to be compared to Na2. For 3 valence electrons represented by N see next post.
What stands out is the atomisation energy of He2 to be compared with that of H2 both with R=0 and maximal E. H2 with E=0.17 is the accepted observed standard value, while it is commonly claimed that He2 cannot form because E<0.
RealQM gives E=0.38 for He2 indicating that He2 can form (maybe under pressure).
So we have here a neat case to test RealQM: Does He form a He2 molecule, or not?
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