Structural, Electronic, and Magnetic Characterization of Ag_nV (n = 1–12) Clusters via DFT and CALYPSO Search

Abstract

Using density functional theory (DFT) combined with the CALYPSO crystal‑structure prediction scheme, we investigated the ground‑state geometries, electronic spectra, and magnetic moments of neutral Ag_nV clusters for n = 1–12. Geometry optimisation reveals that a single V atom preferentially occupies the site with the highest coordination in low‑energy Ag_nV clusters. Replacing an Ag atom by V in Ag_n+1 (n ≥ 5) significantly alters the host cluster’s structure. Simulated infrared, Raman, and photoelectron spectra provide fingerprints for future experimental identification of the most stable isomers. Binding‑energy, dissociation‑energy, and HOMO–LUMO gap analyses show that V incorporation generally stabilises the clusters (except Ag₂V), with the icosahedral Ag₁₂V exhibiting the highest dissociation energy. Magnetic‑moment calculations indicate that the total moment arises almost exclusively from the V atom, decreasing from 5 µ_B in AgV to 1 µ_B in Ag₁₂V; this trend is attributed to charge transfer between V and Ag.

Background

Silver clusters have long attracted interest due to their unique optical and catalytic behaviour [1–20]. Introducing a foreign atom can dramatically reshape a cluster’s properties [21–44], opening pathways for tailored optical, electronic, and magnetic functions in nanomedicine, sensing, and energy devices [45–55]. While V‑doped silver clusters have been studied for their catalytic potential [56–59], comprehensive theoretical data on neutral Ag_nV clusters (n = 1–12) remain sparse, especially concerning their vibrational and electronic spectra and magnetic characteristics. Here we provide a systematic DFT‑based study of Ag_nV (n = 1–12) to fill this gap and guide future experimental work.

Methods

Exchange‑correlation energies were benchmarked against the Ag₂ dimer using the PW91PW91/LanL2DZ level, yielding results in close agreement with experiment [61,62]. The same functional was applied to all Ag_nV clusters, and five additional functionals were tested on AgV, all favouring identical spin states. CALYPSO was employed to generate a diverse set of initial geometries; each was optimised for all relevant spin multiplicities. Convergence criteria were set to 6.0 × 10⁻⁵ Å (displacement), 1.5 × 10⁻⁵ Hartree/Bohr (force), and 1.0 × 10⁻⁶ Hartree (energy).

Results and Discussions

Geometrical Structures and Vibrational Spectra

Extensive structure searches yielded the lowest‑energy isomers for each Ag_nV cluster (n = 1–12). In all cases, the V atom occupies the most highly coordinated site, and for n ≥ 5 the ground‑state adopts a three‑dimensional geometry. The lowest‑energy Ag_nV structures differ markedly from the corresponding Ag_n+1 clusters, reflecting the influence of V on the bonding network. Figures 1 and 2 illustrate these isomers and the growth sequence from trapezoidal to icosahedral motifs. Infrared and Raman spectra (Figure 3) reveal characteristic Ag–V stretching modes and can serve as experimental signatures for cluster identification.

Electronic Properties

Vertical ionization potentials (VIP) and electron affinities (EA) were computed using single‑point energies of the cationic and anionic species, respectively. Calculated VIPs and EAs for Ag_n+1 clusters agree with available experimental data, validating our computational approach. The AgV dimer displays the largest VIP and smallest EA, indicating a hard, electron‑poor species, while the icosahedral Ag₁₂V has the highest EA, signalling readiness to accept an electron. Simulated photoelectron spectra (Figure 4) span 5.5–12 eV and provide fingerprints for future photoelectron experiments.

Average binding energies per atom increase monotonically with cluster size; Ag_nV clusters are generally more strongly bound than their Ag_n+1 counterparts for n ≥ 2, with a pronounced increase for planar to three‑dimensional transitions (Figure 5). Dissociation‑energy analysis (Table 4) identifies Ag_nV → Ag + Ag_n−1V (n = 1, 4–12) and Ag_nV → Ag₂ + Ag_n−2V (n = 2, 3) as the most favourable channels. The largest minimum dissociation energy (2.54 eV) occurs for Ag₁₂V, underscoring its exceptional stability.

The HOMO–LUMO gap shows an odd–even alternation in pure Ag clusters, while V substitution generally reduces the gap for odd‑n clusters, reflecting an open‑shell electronic structure (Figure 6). For even‑n clusters, the gap is largely unaffected by V doping.

Magnetic Properties

Magnetic moments of the ground‑state Ag_nV clusters decrease smoothly from 5 µ_B in AgV to 1 µ_B in Ag₁₂V (Figure 7). Spin‑density‑of‑states plots (Figure 8) show that the dominant contribution originates from the V 3d orbitals, while Ag atoms contribute minimally. Natural bond orbital analysis reveals a decreasing local moment on V with increasing cluster size, ranging from 4.18 µ_B in AgV to 2.08 µ_B in Ag₁₂V. Charge‑transfer calculations (Figure 9) demonstrate that 0.35–2.92 e transferred from Ag to V in three‑dimensional clusters, correlating with the observed decline in magnetic moment (Figure 10). Thus, the magnetic behaviour is governed by V–Ag charge redistribution.

Conclusions

DFT and CALYPSO investigations reveal that V atoms occupy the most highly coordinated sites in Ag_nV clusters, altering the host geometry for n ≥ 5. The simulated IR, Raman, and PES spectra provide experimental fingerprints for future studies. V incorporation generally stabilises the clusters (except Ag₂V) and increases dissociation energies, with the icosahedral Ag₁₂V being the most robust. Chemical reactivity, inferred from HOMO–LUMO gaps, is higher for odd‑n clusters. Magnetic moments are dominated by the V atom, decreasing from 5 µ_B to 1 µ_B as n grows, a trend driven by charge transfer from Ag to V.