DISTANCE-BASED INDICES COMPUTATION OF SYMMETRY MOLECULAR STRUCTURES

Most of molecular structures have symmetrical characteristics. It inspires us to calculate the topological indices by means of group theory. In this paper, we present the formulations for computing the several distance-based topological indices using group theory. We solve some examples as applications of our results. Mathematics Subject Classification: 05C70.


Introduction
In early years, many chemical experiments showed the evidence that the biochemical properties of chemical compounds, materials and drugs are closely related to their molecular structures.As a result, topological indices are introduced as numerical parameters of molecular graph, which play a vital role in understanding the properties of chemical compounds and are applied in disciplines such as chemistry, physics and medicine science.In chemical graph theory, a molecular structure is expressed as a molecular graph G in which atoms are taken as vertices and chemical bonds are taken as edges.A topological index can be considered as a function f : G → R + .In the past 40 years, scholars introduced many topological indices, such as Wiener index, Zagreb index, harmonic index, sum connectivity index, etc which reflect certain structural characteristics of organic molecules.There were many works contributing to report these distance-based or degree-based indices of special molecular structures (See Farahani et al. [1], Jamil et al. [2], Gao and Farahani [3], Gao et al. [4,5,6] and Gao and Wang [7,8,9] for details).The notation and terminology that were used but were undefined in this paper can be found in [10].One of oldest indices, the Wiener index was defined as the sum of distance for all pair of vertices, The modified Wiener index was introduced by Nikolić et al. [11] as the extension of the Wiener index which was defined as Several conclusions on modified Wiener index can be referred to Vukićević and Źerovnik [12], Vukićević and Gutman [13], Lim [14], Gorse and Źerovnik [15], Vukićević and Graovac [16], and Gutman et al. [17].Moreover, the hyper-Wiener index and λ-modified hyper-Wiener index are defined as and respectively.Some important contributions on hyper-Wiener index can be found in Gutman [18], Gutman and Furtula [19], Eliasi and Taeri [20,21], Iranmanesh et al. [22], Yazdani and Bahrami [23], Behtoei et al. [24], Mansour and Schork [25], Heydari [26], Ashrafi et al. [27], and Heydari [28].
Its corresponding Harary polynomial can be defined as The second and third Harary indices are defined as More generally, the generalized Harary index was introduced by Das et al. [31] which is defined as where t ∈ N is a non-negative integer.Hence, Harary index is a special case of generalized Harary index when t = 0.One topological index related to Wiener index is the reciprocal complementary Wiener (RCW) index which is defined by Zhou et al. [32] and can be defined as where D(G) is the diameter of molecular graph G.In what follows, we always denote D(G) as the diameter of molecular graph G. Furthermore, the multiplicative version of the Wiener index was defined by Gutman et al. [33,34] as The logarithm of multiplicative Wiener index was defined as So far, many mathematical approaches are given to calculate different topological indices and received good results.Since most of the chemical compounds have symmetric structures.It inspires us to consider the computation of topological indices by using group theory.We use the automorphism groups and its orbits to simplify the computation of molecular graphs for some distance-based indices.

Main results and proofs
To discuss the symmetry molecular structures, we should first introduce symmetry operations which are defined as operations that move a fixed molecule structure from a previous condition to another, and any two states can't be differentiated from each other.Obviously, all the symmetry operations on a molecular structure constitute a group which is called the point group of the molecular structure.When an element p of point group P (i.e., a symmetry operation on the molecular structure) operates on the molecular graph, it provides the vertices of the molecular graph a permutation.We denote p(v) as the image of vertex v under the operation p.If there exists a p ∈ P that satisfies p(v) = u for two vertices v and u, then we define an equivalence binary relation denoted by v ∼ u.By means of this equivalent relation, the vertex set is divided into several equivalence classes: The group called transitive if it has only one orbit, and it is called intransitive otherwise.Moreover, we can define the orbits of subgroup H in similar way which could also be either transitive or intransitive.Set Hence, we have Following theorem is about the calculation of different topological indices when the point group is not necessarily transitive.
Theorem 2.1.Let H P G be a subgroup of P G , and Proof.We only prove for W λ (G).The remaining cases can be proved in similar fashion.Since W λ (u, G) is equal to the sum of all vertices in the same orbit, we infer Hence, in terms of (1), Hence, the desired result is obtained.
The next result is about the computation of topological indices when the point group of the molecular graph is transitive.
Lemma 2.2.If the point group P G of the molecular graph is transitive.Then for any v ∈ V (G), we have In terms of Lemma 2.2, to calculate the distance-based topological indices of the molecular graph with transitive point group, we only need to choose any vertex v ∈ V (G) and calculate the distances between v and u ∈ V (G) − {v}.Take a subgroup H of P G , which is not necessarily transitive even if P G is transitive.Now, the vertex set V (G) can be divided into orbits of For any i and any z ∈ Θ i , there exist Consequently, we yield The remaining parts follows similarly, hence, we complete the proof.
In theorem 2.3, we can see that for a vertex v 1 ∈ Θ 1 , we do not need to compute all distances between v 1 and V (G) − {v 1 }.It is enough to select one vertex from Θ i .In real practice, we select a subgroup H so that Θ 1 is as small as possible in order to simplify the calculation.Specially, if |Θ 1 | = 1 (H fixes v 1 ), we only count r − 1 times.Hence, we can give the following corollary.
In order to reduce the computation steps of the distance-based topological indices, note that a large number of the molecular structures have the layered structure such that the different orbits have consecutive distances from a fixed vertex.In such a situation, we have following theorem.
Theorem 2.5.Assume that P G is transitive and Proof.Science the molecular graphs are connected and all the elements in the same orbit have equal distances from v 1 , the orbits saturate the vacancy between Θ 1 and Θ k by means of their distances from v 1 .Since only r − 1 orbits different from Θ 1 and d(v 1 , v k ) ≥ r − 1, we infer that the orbits run consecutively between Θ 1 and Θ k , which reveals that the vertices The remaining cases can be easily proved in similar fashion.

Computation Examples
In this section, we give five illustrative examples to explain our method.In the following contexts, we always assume that n is the number of vertex in molecular graph G and the regular polyhedrons meet the conditions of the theorem 2.5.
Example 3.1 (Computation on tetrahedron).The structure of tetrahedron (denoted by G 1 ) can refer to figure 1.Let P G1 be its point group.We need first determine the subgroup R P G1 of all the rotation in P G1 .The elements consisting of R are as follows: (1) the identity; (2) rotations through the angle π about each of three axes joining the midpoints of opposite edges; (3) rotations through angles of 2π 3 and 4π 3 on the each of four axes joining vertices with centers of opposite faces.So, we have |R| = 12.Clearly, R and P G1 are transitive.Select Example 3.2 (Computation on cube).The structure of cube (denoted as G 2 ) can refer to figure 2. In this case, the subgroup R P G2 of all the rotations consists of the follows: (1) rotations through the angle π on each of six axes joining midpoints of diagonally opposite edges; (2) rotations through angles of π 2 and 3π 2 about each of four axes joining extreme opposite vertices; (3) rotations through angles of π 2 , π, and 3π 2 about each of three axes joining the centers of opposite faces.Thus, by simple computation, we get |R| = 24.Clearly, R and P G2 are both transitive.H is selected as in the first instance but the rotations are around the axis joining the two opposite vertices v 1 and v 3 .We get four orbits with representatives v 1 , v 2 , v 3 and v 4 as presented in the figure 2

Conclusion
In this paper, we mainly report the approach on how to use group theory to determine the distance-based topological indices for certain important symmetry chemical structures.Since these Wiener related and other distance-based topological indices are widely applied in the analysis of both the boiling point and melting point of chemical compounds and QSPR/QSAR study, the promising prospects of their application for the chemical, medical and pharmacy engineering will be illustrated in the theoretical conclusion that is obtained in this article.

Figure 1 . 1 H
Figure 1.The structure of tetrahedron G 1

Example 3 . 4 (
Computation on icosahedron).The structure of icosahedron (denoted by G 4 ) can refer to figure 4. The rotation subgroup R of the point group consists: (1)the identity; (2) rotations through the angle π about each of fifteen axes joining midpoints of opposite edges; (3) rotations through angles of 2π 3 and 4π 3 about each of ten axes joining centers of opposite faces; (4) rotations through angles of 2π 5 , 4π 5 , 6π 5 , and 8π 5 about each of six axes joining extreme opposite vertices.Therefore, we have |R| = 60.Furthermore, R and P G4 are