[Pw_forum] How to understand the intermolecular force between water molecules calculated by PWSCF

[Axel Kohlmeyer]

On Sat, 3 Jan 2009, vega lew wrote:

VL> Dear all,
VL> 
VL> if we calculated the a couple of  water molecles in a large enough boxes, we
VL> will find the H of water must point to O of another water.
VL> We often call this kind of attraction as hydrogen bond.
VL> But I want to know except for hydrogen bond, is there van der waals force or
VL> other forces included in the results given by PWSCF?

yes and no. 

PWSCF computes interactions/forces between atoms based on quantum 
mechanics (using DFT), and thus those forces are the result of 
_all_ atoms in a non-additive way, i.e. it contains in principle
everything.

decomposing this into individual (pairwise) contributions (coulomb, 
non-coulomb, bonded, non-bonded, ...) is entirely empirical (or 
phenomenological) and the results depend on your definition of the 
individual contributions. this is what is done in the parametrization of 
classical force fields and thus explains why those parameters are not 
very transfereable. hydrogen bonds are a phenomenoligical concept added 
on top of that and somewhat ill defined at that (there are many 
different definitions of when there is a hydrogen bond between atoms or 
not). the most common definition of a hydrogen bond actually involves 
three atoms, the hydrogen bond donor, the acceptor and the hydrogen that 
is located in between, and to call the resulting "structural feature" a 
hydrogen bond the donor and acceptor have to be close enough and the 
hydrogen must somewhat close to the line connecting those two atoms. in 
classical force fields hydrogen bonds are favored by a combination of 
somewhat strong coulombic attraction balanced with vdW repulsion.

now, it is similarly difficult to determine the contribution assigned
to vdW interactions (except for systems that have bascially no other 
interactions, e.g. Argon) and on top of that you have the problem that
current LDA/GGA functionals underrepresent certain exchange and 
correlation interactions which corresponds to what described as vdW
interactions. you'd have to do prohibitively expensive quantum chemistry
calculations (using e.g. CCSD(T) with very large basis sets) to compute
those accurately.

cheers,
   axel.


VL> 
VL> thanks for reading
VL> 
VL> vega
VL> 
VL> 

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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