ðˈɛɹfoːɹ, sˈɜːkɪt sˈɪmjʊlˌeɪɾɚz nˈoːɹməli jˈuːs mˈoːɹ ɛmpˈɪɹɪkəl mˈɑːdəlz ˈɔfən kˈɔːld kəmpˈækt mˈɑːdəlz ðæt duːnˌɑːt dᵻɹˈɛktli mˈɑːdəl ðɪ ˌʌndɚlˈaɪɪŋ fˈɪzɪks.
Therefore, circuit simulators normally use more empirical models (often called compact models) that do not directly model the underlying physics.