A careful analysis of certain observations concerning the polaron nature of current carriers in rutile is presented. The criticism of our paper [l] given in [2] is shown to be unfounded.KpuTasecmx a a n i e r a~~f i , Some arguments confirming the polaron nature of current carriers in rutile were given in [l]. I n the two first sections of his paper Klinger [2] enumerates the well-known results of small polaron theory. I n Section 3 (see items 1' to 5') he criticizes the analysis of experimental data on rutile given in [l].We shall prove in this paper that all arguments given in 1' to 5' of [2] are misleading and do not invalidate our conclusions obtained in [l].1.' I n 1' one may read that "at T = const, a('P)(w) N (T N Ni (1 -K ) as well as a@p)(w) IV 0'' l ) , where d i p ) is the absorption coefficient for polarons localized on impurity ions, d S p ) the absorption coefficient for free polarons, Ni the impurity concentration, K the compensation degree, and (T the dc conductivity. It means that free carrier absorption and absorption due t o nonionized impurities are both proportional to (T, and if one takes G to be a measure of concentration it is impossible to distinguish between them a t T = const. Separation of these both types of absorption can be achieved in particular by taking into account their different T-dependence. On page 484 in [2] Klinger also mentions about this possibility. It is very strange indeed that he did not notice that precisely this simple method was used in [l], where it was shown that G ( O ) increases with increasing T (whereas diP)(w) should decrease with increasing T). The simultaneous increase of cr and a(w) with T for 300 O K < T < 600 OK in samples with deeper donor levels is shown in our recent papers [3].2'. I n 2 we read that "for the SP Seebeck coefficient in" pure "samples, y@P) = const should be observed a t T > T, ( 5 100 OK), while the experiment shows this rather a t T 2 300 OK". Herewith Klinger refers to papers [4] to [7].We reproduce Fig. 6 from [7] which clearly shows that y in rutile is independent of T till T = 70 to 100 OK (cf. Fig. 1). Thus the T-dependence of y is just the same as it should be in Klinger's opinion for small polarons. 1)The original text quoted from [2] is put in inverted commas.

We show that the conclusion on the breakdown of the standard small polaron theory made recently by E. V. de Mello and J. Ranninger ͓Phys. Rev. B 55, 14 872 ͑1997͔͒ is a result of an incorrect interpretation of the electronic and vibronic energy levels of the two-site Holstein model. The small polaron theory, when properly applied, agrees well with the numerical results of these authors. Also we show that their attempt to connect the properties of the calculated correlation functions with the features of the intersite electron hopping is unsuccessful. ͓S0163-1829͑99͒02014-7͔

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