The dependence of the total electron energy density at the (3,−1) critical point (CP) of the H…O interaction against the interatomic distance $(ECP)$ has been obtained by the addition of the local electron kinetic $(GCP)$ and potential $(VCP)$ energy densities dependences $(ECP=GCP+VCP)$ for a set of 83 X-H…O hydrogen bonds (X=C, N, O). The $ECP$ function has been related to the interaction potential by means of a proportionality relationship $U=\u2212\upsilon \u22c5ECP,$ υ being a positive constant in volume units. Based on the $GCP$ and $VCP$ functionalities, the proposed H…O interaction potential has been successfully checked against several physical and chemical properties. The behavior of the *U* function has been compared to Morse and Buckingham-type potentials, leading to an almost perfect matching between all of them when they were constrained to have the same three parameters: the potential well depth $U0,$ its position $r0,$ and the curvature of the potential function at $r0.$ The resulting $U(r)$ function is simply described by the addition of two exponential terms: $U(r)=49\u200a100\u200aexp(\u22123.6r)\u221211\u200a800\u200aexp(\u22122.73r),$ where *U* is in kJ/mol and *r* is the H…O distance in Å.

## REFERENCES

*Molecular Crystals and Molecules*, edited by E. M. Loebl (Academic, New York, 1973).

*The Atom–Atom Potential Method. Applications to Organic Molecular Solids*, edited by M. Cardona (Springer, Berlin, 1987).

*Atoms in Molecules: A Quantum Theory*, edited by J. Halpen and M. L. H. Green, International Series of Monographs on Chemistry (Clarendon, Oxford, 1990).

^{−1}at $T\u2248100\u200aK$ and atmospheric pressure (Ref. 9) lead to a force constant of the hydrogen bond of $k=22.7\u200aN/m$ $(dHB(O\u2026O)=2.965\u200a\xc5)$ for an experimental frequency value of $\nu \u0304=214.3\u200acm\u22121$ (the strongest intermolecular translational mode). Unfortunately, the equilibrium distance $r0100\u200aK=d(H\u2026O)$ is still unpublished to our knowledge. In order to estimate a value for $r0100\u200aK,$ we have used the experimental neutron data for ice VIII $(D2O)$ at $T=10\u200aK$ and $P=2.4\u200aGPa$ (Ref. 10), and the theoretical crystal-orbital

*ab initio*calculations carried out for ice VIII from $P=0$ to $P=28\u200aGPa$ (Ref. 11). In the whole range of pressures, the theoretical analysis shows very small variations of both the HB angle and the O–H covalent bond distance: $178.7\xb0<\alpha (O\u2013H\u2026O)<179.8\xb0$ and $0.9509\u200a\xc5<d(O\u2013H)<0.9554\u200a\xc5.$ In particular, the structural magnitudes obtained from this theoretical analysis, and corresponding to the unit-cell volumes of both experimental studies (Refs. 9 and 10) are: $\alpha (O\u2013H\u2026O)=179.4\xb0$ and 179.6°, and $d(H\u2013O)=0.9516$ and 0.9522 Å, for $Vexp(P\u22480\u200aGPa,T=100\u200aK)=161.0\u200a\xc53$ and $Vexp(P=2.4\u200aGPa,T=10\u200aK)=146.9\u200a\xc53,$ respectively (Refs. 11 and 12) (looking at the volumes, effects of both pressure and temperature are implicitly taken into account when results are transferred to experimental data, even if the theoretical analysis is carried out varying only the pressure). It means that experimental differences in $dHB(O\u2026O)$ values should be related (in a very good degree of approximation) to differences in $d(H\u2026O)$ distances. As the experimental data of Ref. 10 indicate $dHB(O\u2026O)=2.879\u200a\xc5$ and $d(D\u2026O)=1.910\u200a\xc5,$ the estimated hydrogen bond distance at $P=0\u200aGPa$ and $T=100\u200aK$ $(r0100\u200aK)$ should be close to $d(H\u2026O)=1.996\u200a\xc5.$

*Materials Crystal Chemistry*(Marcel Dekker, New York, 1997), pp. 122–124.

*F*against

*U*, the interaction is repulsive $(F<0)$ when $r<r0$ and it is attractive $(F>0)$ when $r>r0,$ with a maximum value $Fd$ at $r=rd.$

*Materials Selection in Mechanical Design*(Pergamon, Oxford, 1992), pp. 47.