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--- spcat-considerations [2024/07/12 15:29] – [The option line] mueller
+++ spcat-considerations [2024/07/15 14:45] (current) – [Considerations for the intensity cutoffs] mueller
@@ Line 92: / Line 92: @@
 of quantum levels as specified in the intensity and variance files.
 Other temperatures will be calculated individually as specified by TEMP in the intensity file.
-Therefore, it is of utmost importance to take a number of aspects into account.
+Therefore, it is of utmost importance to take a number of aspects into account.\\
+\\
+* **FBGN** should be set to 0 or the lowest allowed value.\\
+\\
+* **FEND** should be chosen high enough, such that the value of **QROT** at the highest temperature
+is converged with respect to the quoted digits. Please note: while it may be fine to choose a value
+higher than necessary, we recommend strongly to avoid values too high by far as peculiarities may occur
+at very high //K// values, in particular at very high //J//.\\
+\\
+* It is **very** important to take care that the energy of the lowest allowed hyperfine state is at zero energy!
+This is particular important in cases of large spin-orbit splitting where the energy of the **unsplit**
+//N// = 0 level will be at energy 0 without any modification! The energies will be too low as a consequence!
+On the other hand, if you start your calculation of, e.g., H<sub>3</sub><sup>+</sup> with
+the //J// = //K// = 0 level at 0, your energies will be too high!\\
+\\
+* If you are fitting multiple isotopologs or one isotopolog with several degrees of hyperfine splitting,
+you should set the weights (WTPL and WTMN) of all unwanted "states" to zero in the variance file.\\
+\\
+* The **maximum //K//** value in the variance file should be high enough, such that the value of **QROT**
+at the highest temperature is converged with respect to the quoted digits. See also the respective remark
+for **FEND**.\\
+\\
+* If you need to take into consideration excited vibrational states, and you do not have sufficient
+accurate information on excited vibrational states, it will be best to determine only //Q//<sub>rot</sub>
+analytically and apply //Q//<sub>v</sub> from a harmonic approximation a posteriori.\\
 \\
 ==== Considerations for the intensity cutoffs ====
- \\
+It is very important to choose a reasonable temperature first. If you have more than one,
+it is useful to determine the intensity cutoffs separately for each case, unless
+the temperatures differ only marginally.
+**STR0** should be chosen low enough that at least the //J// = 1 − 0 transitions show up in the catalog file.
+In the case of molecules in cold environments or if your experimental rotatational temperature is low
+and //K<sub>a</sub>// = 1 is characterized well, **STR0** may be chosen even lower such that at least
+the //J// = 1 − 1 transitions show up in the catalog file.
+**STR1** should be chosen high enough that not too many transitions with large uncertainties occur
+in the catalog file, unless you do want this.
+**Please note:** If the line list is relatively small with respect to the parameter set,
+it may be useful to calculate to a value **FEND** that is much smaller than the one to
+calculate the partition function properly. You can have multiple option lines in your
+intensity file after the dipole moments lines and after one blank line.
+Just exchange the desired line with the one that is currently in row 2.
+Please note also that you may have to balance **STR0** and **STR1** to obtain
+a recent catalog output, in particular in cases, in which the two values differ very much.
+In rarer cases, it may be useful to have **STR1** < **STR0**, mainly if you want to
+have frequencies well beyond the Boltzmann peak at the desired frequency.
+In most cases, you will find that it is useful to have **STR0** < **STR1**.
+\\