haʊˈɛvɚ, ɪɾ ɪz nˌɑːt ˈækjʊɹətli mˈɑːdəld fɹʌm fˈɜːst pɹˈɪnsɪpəlz, ænd sˌoʊ ɹᵻzˈɔːɹt ɪz tˈeɪkən tə fˈɪɾɪŋ ɛkspˌɛɹɪmˈɛntəl dˈeɪɾə.
However, it is not accurately modeled from first principles, and so resort is taken to fitting experimental data.