|
This
Pople notation indicates that the molecular structure is optimized (to a
stationary point, minimum or saddle point) with the combination of method and
basis set to the right of
the // Subsequently the energy is calculated at the
optimized geometry with the method and basis set on the left hand side of the
// The
rational behind this mixed approach is that optimized geometries, being relative
energies, are less sensitive to method and basis set than relative energies of
different stationary
points on the potential energy surface. This mixed level approach can thus be
seen as a compromise to level the errors arising from the geometry effect with
the error from energy difference between different geometries, and thus achieve
the ‘best’ result for
a given amount of computational time. How
to get this ‘optimum’ combination is a matter of
‘experience’ and many different mixes have been
proposed. Frank From:
owner-chemistry+frj==chem.au.dk ^%at%^ ccl.net
<owner-chemistry+frj==chem.au.dk ^%at%^ ccl.net>
On Behalf Of Andrew DeYoung andrewdaviddeyoung()gmail.com Hi, Could you please help me to understand the "double
slash" notation that I see in quantum chemical literature? The
notation is: method/basis set//method/basis set What is the convention -- if any -- for the order of the
calculations, (a) or (b)? (a) Is the calculation before the "//" performed
first, while the calculation after the "//" is performed second?
In other words, "do this [left], then that [right]"? (b) Or is it the other way around, meaning that the
left-hand calculation is performed ON the output of the right-hand
calculation? In other words, somewhat like an operator acting on a wave
function; "do this [left] on the output of that
[right]." Below are two examples from the literature. I try to
reason through the notation based on these examples: (1) Maxwell, Tirado-Rives, and Jorgensen. J. Comput. Chem.
1995, 16, 984-1010 ( https://doi.org/10.1002/jcc.540160807 ). In this
paper, ab initio calculations were performed
to determine torsional parameters for organic molecules and ions. From
the abstract, "The rotational energy profiles were obtained at the
HF/6-31G*//HF/6-31G*." In certain cases, they also used
MP2/6-31G*//RHF/6-31G* or MP2/6-31G*//MP2/6-31G*. I think this
means an initial, unconstrained geometry optimization of the entire
molecule/ion was performed first, followed by a geometry optimization at each
selected, constrained value of the torsional angle. In the case of
MP2//RHF, then, is MP2 used for the initial
optimization and RHF for the scan? Because RHF is less expensive, I
think, that would probably make sense. Based on this, I would say that
MP2//RHF means that MP2 is performed first and RHF is performed second; in other
words, the "double slash" notation
is read from left to right. So interpretation (a) above is
correct. (2) Liptak and Shields, J. Am. Chem. Soc. 2001, 123,
7314-7319 ( https://doi.org/10.1021/ja010534f ). In this paper,
the free energy of solvation, DeltaG_s, is calculated using an implementation
of the polarizable conductor model (CPCM) -- see the third page (p. 7316) of
this paper. Near the top left-hand side of that page, it says, "The
CPCM calculations were performed as SPCs (single-point calculations) using the
6-31G(d) and 6-31+G(d) basis sets
on the HF/6-31G(d) and HF/6-31+G(d) geometries for each of the six
systems." Based on that description, the HF geometry optimization was
performed first and the CPCM single-point calculation was performed
second. So, if my reasoning from example (1) above
is correct (i.e., interpretation (a) above), I'd expect the "double
slash" notation in the paper to be HF//CPCM. But, in fact, it's
not. In the Results section near the bottom of the left-hand side of the
third page (p. 7316), the authors state, "CPCM/6-31G(d)//HF/6-31G(d),
CPCM/6-31+G(d)//HF/6-31G(d), and CPCM/6-31+G(d)//HF/6-31+G(d) are denoted S1,
S2, and S3 [in Table 1]." This, I think, implies that
interpretation (b) above is correct. The strange thing, though, is
that at the bottom of Table 1 on the fourth page [p. 7317],
the definitions of S1, S2, and S3 are different from those stated in the text,
by an "extra" "HF" to the left of the "//"; there,
they state, "S1: CPCM/HF/6-31G(d)//HF/6-31G(d). S2:
CPCM/HF/6-31+G(d)//HF/6-31G(d). S3:
CPCM/HF/6-31+G(d)//HF/6-31+G(d)." It's entirely possible, then, that there is no set
convention for "double slash notation" in quantum chemistry (or that
I'm misinterpreting these papers). Could anyone shed some light on this,
since I'm not a quantum chemist? Thanks so much for any insight you can
provide, Andrew Andrew DeYoung, PhD Carnegie Mellon University |