op95interp_mod.f

OP95INTERP_MOD holds the interpolation stencil and weights for the OPAL 1995 opacity tables: GETOPAL95 computes them for the current $\rho$, $T$, $X$, $Z$, and the interpolators OP952D, OP953D and OP954D apply them. Shared by 6 routines. It replaces three rp_1 blocks, COMMON/OP95INDX/ (the indices), COMMON/OP95FACT/ (the weights) and COMMON/OP95EXT/ (extrapolation), first merged into one COMMON/OP95INTERP/ and then made a module (MHP via Claude, 10/9–10/14/26); OP95EXT's RL and RLL are renamed RHO_T63L and RHO_T63LL, since RL means $\log r$ everywhere else in the code.

What its variables hold, who sets them, who reads them. All are set by GETOPAL95 on each call; the interpolators read them.

• JZ, JX(4,4), JT, JD(4): the first table index of each four-point stencil, in $Z$, in $X$ (for each of the four $Z$), in $T$, and in $R$ (for each of the four $T$, since the stencil in $R$ may shift at high density). Initialized to 1, the values of LL95TBL's DATA statement at rp_1.
• BZ(4), BX(4,4), BT(4), BD(4,4): the four-point weights in $Z$, $X$, $T$ and $R$ (INTRP2 for $Z$ and $X$, INTERP for $T$ and $R$); BTT(4), BDD(4,4): INTERP's weights for the derivatives in $T$ and $R$.
• RHO_T63L: $\log R = \log\rho - 3(\log T - 6)$, the table's density variable; RHO_T63LL: the table's lowest $\log R$, used in place of RHO_T63L when extrapolating below the table.
• LEXTL: extrapolation below the table in $R$; LEXTH, LEXH(4): extrapolation above it (the empty high-density corner), overall and for each of the four temperatures; RLH(4): the table edge in $\log R$ at each temperature, from which the extrapolation is linear.

A blanket USE OP95INTERP_MOD in each routine; the main program and LL95TBL use it but refer to none of its names. The module's header says the four initialized indices came from PARMIN's DATA statements and are namelist variables; they came from LL95TBL's DATA statement and are in no namelist.