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370 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 3: POLYMERS & OTHER MATERIALS can be obtained experimentally. For instance, the relative thermodynamic stability of different surface reconstructions of a polar surface can be better determined by theory than by any experimental technique. Or, as another example, a detailed understanding of the bonding between oxide substrate and adsorbed atom, molecule, or radical can only be achieved by analyzing reliable quantum chemical wave functions. In many cases researches are in urgent need of theory, since many experimental observations, in particular in the field of optical and electron spectroscopy, can hardly be interpreted correctly without applying detailed theoretical calculations. Recent progress attained in the methodology of electronic structure calculations and in computer technology has made quantum chemical ab initio calculations inexpensive and reliable. They are indeed a helpful interpretative tool even for systems as complex as oxide surfaces. Increased numbers of papers containing both experimental and theoretical results confirm this fact (Freund et al. 1996, 2001; Eglitis et al. 2003). 7. SUMMARY To summarize the results presented in this work, in recent years we have made considerable progress in elaborating new sensor materials and in optimizing those already known. However, in spite of the pro- gress achieved, many problems discovered even early in the development of chemical sensors have not yet been resolved. This means that design of new sensor materials and technologies will continue, since