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Classical finite element methods are only capable of describing a limited computation area; the substrate of a micro-acoustic device must therefore be described by suitable boundary conditions. This is obtained by absorbing boundary conditions (ABC) or perfectly matched layers (PML) which both suppress reflections from the substrate. PML was implemented into a high-performance finite element code. The employed variant of the PML approach relies on a complex variable transformation of the basic piezoelectric equations in the PML layer. Harmonic admittances of a system with aluminum electrodes on a 42deg YX- LiTaO3 substrate are determined with PML and ABC methods and compared to FEM/BEM results, which can be considered as exact solution of the piezoelectric half space problem. The results of PML approach FEM/BEM, while the ABC results deviate. The correct adjustment of PML parameters to minimize reflection at the substrate/PML interface is illustrated by visualizations of the wave fields.