DFT-Based Studies On Atomic Clusters

An assembly of a few to thousands of atoms or molecules is referred to as a cluster. This chapter exclusively deals with atomic clusters and their classifications based on the nature of bonding and the number of constituents. The size effect, surface phenomena, and variation of properties with the size of atomic clusters have also been discussed. The difficulties in cluster experiments have been highlighted. Various theoretical methods to study the clusters have been mentioned, with the main focus on the density functional theory (DFT). The accuracy of various DFT methods and choosing appropriate methods have been particularly discussed. Finally, the way to perform DFT-based studies on clusters is also explored. This chapter provides a brief introduction to what the book is all about.

This chapter exclusively addresses the algorithms employed to perform geometry optimization of clusters. These algorithms can be broadly classified into two groups: gradients-based algorithms and gradient-free algorithms. Gradient-based algorithms use the gradient of potential energy functions to give local minima. On the contrary, gradient-free algorithms are inspired by natural processes, which exploit some mathematical models, which lead to global minimum. Although there are a variety of gradient-free algorithms, some of the most popular ones include genetic algorithm, particle swarm, simulated annealing, etc. The strengths and weaknesses of all these algorithms have been also discussed.

Activation of CO2 is the first step towards its reduction to more useful chemicals. Electrochemical CO2 reduction reactions can lead to high value-added chemical and materials production while helping decrease anthropogenic CO2 emissions. In studies, it was found that copper metal clusters can reduce CO2 to more than thirty different hydrocarbons and oxygenates, yet they lack the required selectivity. Density functional theory (DFT)-based studies are carried out on copper clusters, doped clusters, nano-structures and Cu-based alloy catalysts to assess the activity and selectivity of CO2 reduction to generate carbon monoxide (CO), formic acid (HCOOH), formaldehyde (H2C=O), methanol (CH3OH) and methane (CH4 ). In this chapter, we will discuss the effect of the adsorption of CO2 on (Sc, Ti, V) metaldoped clusters. DFT studies carried out for these clusters showed a high CO2 adsorption energy, a low activation barrier for its dissociation, and a facile regeneration of the clusters. The reaction energies (dopant-dependent), the mechanisms for reaction, dissociation barriers for CO2, and less desorption energies (dopant dependent) for carbon monoxide (CO) during the activation of CO2 with Cu3X clusters (X= first row transition metals) are discussed in the chapter. C6Li6 is not capable of capturing CO2 molecules but is effective in their storage. The interaction of CO2with superalkalis such as FLi2 , OLi3 , and NLi4 and non-metallic superalkalis such as F2H3 , O2H5, and N2H7 is also included due to its applications.

“Endohedral metallofullerenes (EMFs)”, also known as metallofullerenes, are the hybrid molecules of spherical nanocarbons. The structure of EMFs consists of atomic metal(s) or metal-containing clusters enclosed inside the fullerenes. These unique EMFs have potential applications in various fields. Smalley and coworkers first reported the EMFs in 1985 [1], soon after C60 was discovered. In this chapter, we will discuss the computational studies carried out for the various types of EMFs and their potential role in industrial applications. Endohedral metallofullerenes M@C44 containing several different gap atoms of scandium and titanium family are experimentally detected. Dispersion-corrected DFT was carried out to study C44 and its fullerene derivatives to calculate the binding properties and interactions. Results showed that the computational and experimental studies agreed well with C44 and its endohedral compounds. Results predicted that C44 D2 (89) isomer is suitable for forming endohedral compounds. The binding affinity of the endohedral atom and its enclosure showed good profusion of these clusters. The DFT studies are used to study different characteristics of the doped C44-D2 (89) to the tri and tetra anions of empty C44-D2 clusters. These metal−cage bonds possess partial (ionic and covalent) interactions. The aromaticity of the cage is estimated with nucleus-independent chemical shift (NICS(0)), as it plays a vital role in balancing the endohedral species.

Metal clusters, atomically precise aggregates of metal atoms, when prepared in the gas phase, produce only “magic numbers” sizes. Small-sized metal clusters (2 nm (<~100 atoms)) have different geometric and electronic properties as compared to bulk metal structures. Metal clusters are used as new functional nanomaterials in various applications. In this chapter, the interactive properties of different nucleic acid bases (NAB) with M2 /M2 2+ (M = Ag, Au, and Cu) are studied by theoretical means. Transition metals have a great affinity toward nucleic acids, which makes them suitable candidates for metal ion sensing, removal of toxic metal ions, and the construction of functional metal nanostructures. The highly significant symmetric dinuclear metalmediated homo base pairs structures with significant stabilities play a vital role in various applications. Metal-nucleobase complexes have wide applications, such as sensors, bidirectional nucleobases, logic gates, and nanowires. A logic gate programs cells for biomedical and environmental applications. In this chapter, the electronic and optoelectronic properties of M2 and M2 2+ with various NAB pairs, such as adenine−thymine (AT), adenine−uracil (AU), adenine thymine (ATAT) Watson Crick (WCST) stacking pairs, adenine−adenine stacking pairs (AAST), and adenine−adenine hydrogen bonding (AAHB) (M = Ag, Au, Cu), are studies, along with their applications. 

This chapter is exclusively devoted to molecular clusters. Molecular clusters refer to finite aggregates of molecules or compounds. In such clusters, molecules are usually bound by hydrogen bonding, van der Waals, or similar weak interactions. We have shown some examples of molecular clusters, such as the clusters of H2O and LiF molecules. Alkaline-earth metal oxide clusters such as (BeO)N , (MgO)N , and (CaO)N have specifically been discussed, and their hydrogen storage behavior have been explored using various DFT methods such as B3LYP, PBE-D3, M06-2X, etc. B3LYP is a hybrid functional. PBE-D3 is a dispersion-corrected GGA functional, whereas M06-2X is a meta GGA functional, as discussed in Chapter 1. It was found that small (BeO)N clusters prefer planar structures due to partially covalent Be-O bonds, unlike ionic Mg-O and Ca-O bonds. Although all these clusters can effectively adsorb H2 molecules, smaller clusters serve as better adsorbents due to the high percentage mass ratio.

This chapter provides a summary of the book and offers some future perspectives.