Figure 18: Cohesion energy calculations. (a) An elementary unit of La2CuO4 in the tetragonal structure that includes a CuO6 octahedron and two La atoms is shown. The oxygen atoms in the CuO2 planes are labeled as O( ), and-out-of plane oxygen atoms are labeled as O( ). O( ) atoms are counted as 1/2 in the summations, since they are shared by two units. La2CuO4 in the tetragonal structure can be described by four structural parameters; distance between CuO2 planes ( ), Cu–O( ) distance ( ), Cu–O( ) distance ( ), and Cu–La distance ( ). In the excited state, the valence of a Cu atom is decreased by , and the valence of an O( ) atom is increased by , where is the number of photons absorbed per copper atom. (b) Calculated cohesion energy as a function of the c-axis constant in the ground ( ) and excited states with assuming a uniform expansion along the c-axis direction. Calculated cohesion energy in the excited state is accurate up to an overall constant. (c) Comparison of the calculated using different methods with the experiment (blue points). The dashed green line is the result of the calculation without considering the van der Walls interactions; dashed red and black lines are the calculated after taking van der Walls interaction into account and assuming uniform expansion or independently variable parameters, respectively (see text). All of the calculations reproduce the magnitude of the observed expansion remarkably well.