The Aqueous Ca²⁺ System, in Comparison with Zn²⁺, Fe^{3 +}, and Al^{3 +}: An Ab Initio Molecular Dynamics Study

Abstract

Herein, we report on the structure and dynamics of the aqueous Ca(2+) system studied by using ab initio molecular dynamics (AIMD) simulations. Our detailed study revealed the formation of well-formed hydration shells with characteristics that were significantly different to those of bulk water. To facilitate a robust comparison with state-of-the-art X-ray absorption fine structure (XAFS) data, we employ a 1st principles MD-XAFS procedure and directly compare simulated and experimental XAFS spectra. A comparison of the data for the aqueous Ca(2+) system with those of the recently reported Zn(2+), Fe(3+), and Al(3+) species showed that many of their structural characteristics correlated well with charge density on the cation. Some very important exceptions were found, which indicated a strong sensitivity of the solvent structure towards the cation's valence electronic structure. Average dipole moments for the 2nd shell of all cations were suppressed relative to bulk water.

Effects of H-bond asymmetry on the electronic properties of liquid water – An AIMD analysis

“…The average values of the distributions are rather different, namely about 3.00 D from the Wannier scheme and 2.40 D from the Bader scheme. The published values for the average Wannier-type dipole moment in liquid water are generally reported to lie close to 3.0 D for various AIMD simulations using PBE or BLYP-based functionals in the literature (a selection of references is found in [61][62][63]). On average the water molecule is thus enhanced by a factor of more than 60% compared to the gas-phase value.…”

Large polarization but small electron transfer for water around Al3+ in a highly hydrated crystal

“…In the cited papers, the dipole moment values were always obtained from a Wannier center analysis calculated from the electronic wavefunction for selected snapshots from CPMD simulations using the PBE or BLYP functionals. Thus, Bogatko et al 12 also found a value of 2.8 D for K + (aq). There exist some additional papers in the literature where dipole moments for first-shell water molecules around cations have been calculated, but they have not been included in our graph since the OÁ Á ÁO distances between the first and second hydration shells were not reported or they were not easily calculated (by us) from the published data.…”

Ab initiomolecular dynamics studies of hydroxide coordination of alkaline earth metals and uranyl

“…61 The quantum chemical statistical mechanical (QMSTAT) study by Tofteberg et al 58 reported a CN of 6.9 with a Ca-O length of 2.5 Å. Various CPMD 41,42,62,63 studies of the hydration structure of Ca 2+ calculated CN in the range 5.9 41 to 7.2 62 and Ca-O distances of 2.36 41 to 2.45 Å. 63 A BOMD study by Mehandzhiyski et al 64 found a CN of 7 with a Ca-O distance of 2.45 Å, in agreement with earlier CPMD studies.…”

“…Each calculated trajectory was 20 ps long and was comprised of 40 000 steps, each of length 0.5 fs. The first 5 ps of each trajectory was treated as an equilibration period, in keeping with previous studies, 41,63,100,101,[138][139][140] and was not considered in subsequent analysis. It is worth noting that an alternative approach of performing initial classical MD simulations in the NPT ensemble using classical MD may lead to better equilibration.…”