The guanidine and maleic acid (1:1) complex. The additional theoretical and experimental studies

► The crystal of guanidinium × maleic acid can be used in NLO optics as second order generator of light. ► The HOMO-LUMO analysis suggests that guanidinium cation should be flat. This result confirms previous study of many guanidinium compounds. ► On the basis of experimental data for both forms of guanidinium × maleic acid complex during the theoretical studies only one equilibrium energy structure was found.