Keywords

1 Introduction

In recent years, Lithium is found to be form stoichiometric polymer with various elements. On the other hand, There are a lot of practical application of fluoride, such as the six lithium fluoride phosphate is the core of the electrolyte materials, and is one of the key materials necessary for the lithium battery electrolyte; LiF and other electronic injection material introduction of organic optoelectronic devices have become a good luminescent material [1,2,3,4]. F-Li2 Polymer belongs to super valence compounds containing odd electronic, it has good nonlinear optical properties, so the scientists study on super molecular structure of alkali metal fluoride has always maintained a strong interesting in F-Li2[5,6,7].

When we study reaction kinetics characteristics, the first thing is to build precise PES. In the past ten years, some studies polarization molecular science of the system offers F-Li2 polymer structure and the dynamic response process [8,9,10,11]. Through investigation we learned that most of the potential energy surface of F-Li2 polymer before, is the method by semi-empirical fitting.

Our calculations are covered a wide range of interaction energy of the potential energy surface. First, considering vibrational weakly bound van der Waals complexes and the good performance on similar optimization, we used the CCSD (T) calculation method for single point of interaction energy. And then we described the features of the F-Li2 PES. At last we focus our attention on the ground state energy of this system.

2 Ab Initio Calculations

When we do some calculation for alkali metal diatomic molecules the electronic related functions must be considered. The basis sets used for frequency calculations consist of aug-cc-pCVQZ for the Li atom and aug-cc-pCVDZ for the F atom. At the same time, we added with an additional (3s3p2d) set of midbond functions. In order to improve the convergence of basis set, we joined Midbond functions (mf) at the midpoint of R. We used quantum analysis framework in the process of computing the Jacobi coordinates system (r, R,\(\theta\)). As shown in Fig. 1. The r is the distance of Li-Li, the R is the length of the vector connecting the Li-Li center of mass and the F atom, and \(\theta\) is the angle between R and the x axis. For a given value of R, the angle \(\theta\) changes from 0° to 90° in steps of 10°. We calculated 365 geometries for the whole interaction energy.and the ground state of the spacing is req = 2.696 \(a_0\) [12].

To ensure that the basis permits polarization by Li, we added diffuse augmentation functions. In the well range (the short range) (\(0a_{0} \le R \le {4}a_{0}\)), while \(\theta { = }0^{\text{o}}\) and \(\theta { = }90^{\text{o}}\),we used the interval equal step way \(\Delta R = 0.{1}a_{0}\). In the long range (\({4}a_{0} \le R \le {11}a_{0}\)), with \(\Delta R = 1a_{0}\).

Fig. 1.
figure 1

Jacobi coordinates system

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The ab initio calculations have been calculated with Gaussian 09W perform packet [13]. We considered all electronic correlation calculation process. The method of supra-molecular was used when we calculated the interaction between Alkali metal pairs to the atom fluoride.

3 Results and Discussion

We show the behavior of the potential energy surface from ten different anglers as we can see In Fig. 2(a). When \(R < 2a_0\), with the increase of R ten different points of view of potential energy are gradually increase. After reaching different peaks the potential energy reducing with R increasing.In the scope of \(R > 5a_0\) the potential energy changes flatten. In Fig. 2(b) We can clearly see that an obvious potential barrier appears at \(\theta\) = 30° and at \(\theta\) = 90° a shallow potential well appears about the range (\({1}{\text{.8}}a_{0} \le {\text{R}} \le {2}{\text{.2}}a_{0}\)).

Fig. 2.
figure 2

Orientational features of the potential energy surface of F-Li2.

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Fig. 3.
figure 3

Contours of the V00 PES for F-Li2 polymer

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In Fig. 3 we can see clearly that as the R increasing in the large area of the long-range the interaction converge to the same asymptotic value. The shape of a “T” backwards Li–F–Li is the lowest energy configuration (–3.87eV(–1.763e–5Hartree) at R = 2a0).

In Fig. 4 we show the 3D-PES for angles \(\theta = 0^{\circ}\!{-}360^{\circ}\). The figure shows that the potential energy changes present strong anisotropy. The saddle point is located at R = 2.6Å and \(\theta\) = 0°. Clearly we can see that a shallow well appears at \(\theta\) = 90°. The absolute dissociation energy we can get is –3.87eV(–1.763e–5Hartree), which is close to that obtained from the experiment [14]. This result reflected the potential energy changes in large angle is anisotropic.

Fig. 4.
figure 4

PES for the Li-Li-F (angle \(\theta = 0^{\circ}\!{-}360^{\circ}\))

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In Fig. 4, there are two obvious peaks on the ground state potential energy surface. Peak corresponds to the left is F + Li2 and the right peak corresponds to the Li - F - Li reactants. We can easily see the whole potential energy is anisotropic.

4 Concluding Remarks

We adopted ab initio calculation method to calculate the ground state potential energy of F-Li2 polymer. By the continental scientific drilling (CCSD (T) method and aug-cc-pCVQZ /aug-cc-pCVDZ + 332 basis set, we draw out the potential energy surface in the whole process of the three dimensional space. Compared with previous two-dimensional potentials with fixed re = 2.696a0, Our theoretical results agree well with the experimental data.