Ingo Fischer opened a general discussion of the paper by Anne Zehnacker: You also presented data on the structure of the ionic dipeptide and discuss the relative stability of the isomers in terms of the CH-p and NH-p bonds in the ion. But in the ion you also have ion-multipole interactions. Can you comment on their role for the structure and stability of the dipeptide isomers? Anne Zehnacker-Rentien responded: Actually, the CH and NH p-bonds disappear in the ionic "folded-extended" structure. Indeed, most of the charge is borne by the extended aromatic ring. Therefore, the interaction between the charged benzene ring and the NH becomes repulsive. This is why the extended ring undergoes a rotation upon ionisation, to favour an interaction between CO and the aromatic ring. However, I agree that this interaction is probably more of a dipole/charge interaction than a COp interaction. Ingo Fischer added: You point out that the NH-p bond is weaker in the c-LL isomer than in the c-LD isomer, while for the CH-p bond you observe the opposite trend. Do you have a simple picture that explains this result? Anne Zehnacker-Rentien replied: In general, steric constraints due to the cyclic nature of the dipeptide add to chirality effects and make these effects more prominent. This is the case for example for 1-amino-2-indanol in which the (R,S) and (R,R) diastereomers show much more structural differences than linear 1,2 amino alcohols. 1 In the cyclo Tyr-Tyr studied here, the cyclic nature of the peptide backbone is also responsible for the differences in interactions. Because of the different position of the extended aromatic ring in the "folded/extended" geometry of the two diastereomers (axial for L and equatorial for D), the CH-p

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Water clusters provide an opportunity to explore the effects of nuclear quantum effects on the stability of hydrogenbonded networks. Because of their well-defined size, such studies provide an opportunity to make detailed comparisons between experiment and calculation and explore experimental measurable consequences of nuclear quantum effects through explorations of changes in zero-point structures upon partial or full deuteration. Such studies are challenging due to the large number of relevant degrees of freedom, as the changes in the anharmonic frequencies of OH stretching vibrations upon hydrogen bond formation are comparable to the zero-point energy in the low-frequency intermolecular vibrations. This also makes these systems challenging for calculations by Diffusion Monte Carlo (DMC). Earlier studies indicated that ensemble sizes of upwards of 10 6 walkers are needed to obtain reliable energies. aIn this presentation, we will describe recent work in our group in developing a modified DMC approach in which guiding functions are used to improve the sampling of the high-frequency intramolecular vibrations (OH stretches and HOH bends). With this modification, the energies and wave functions can be obtained using ensembles that are more than an order of magnitude smaller compared to those used in earlier studies. b With this more efficient algorithm, we explore the effects of nuclear quantum effects on the relative energies of the low-energy prism and cage structures, as well as the relative energies of the partially deuterated (H 2 O) 5 (D 2 O) and (H 2 O)(D 2 O) 5 clusters.

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