\[\frac{NdChar}{1 + e^{\frac{-\left(\left(\left(Ec - Vef\right) - EDonor\right) - mu\right)}{KbT}}} + \frac{NaChar}{1 + e^{\frac{\left(\left(Ev + Vef\right) + EAccept\right) + \left(-mu\right)}{KbT}}}\]

\frac{NdChar}{1 + e^{\frac{-\left(\left(\left(Ec - Vef\right) - EDonor\right) - mu\right)}{KbT}}} + \frac{NaChar}{1 + e^{\frac{\left(\left(Ev + Vef\right) + EAccept\right) + \left(-mu\right)}{KbT}}}

double f(double NdChar, double Ec, double Vef, double EDonor, double mu, double KbT, double NaChar, double Ev, double EAccept) {
        double r8148 = NdChar;
        double r8149 = 1.0;
        double r8150 = Ec;
        double r8151 = Vef;
        double r8152 = r8150 - r8151;
        double r8153 = EDonor;
        double r8154 = r8152 - r8153;
        double r8155 = mu;
        double r8156 = r8154 - r8155;
        double r8157 = -r8156;
        double r8158 = KbT;
        double r8159 = r8157 / r8158;
        double r8160 = exp(r8159);
        double r8161 = r8149 + r8160;
        double r8162 = r8148 / r8161;
        double r8163 = NaChar;
        double r8164 = Ev;
        double r8165 = r8164 + r8151;
        double r8166 = EAccept;
        double r8167 = r8165 + r8166;
        double r8168 = -r8155;
        double r8169 = r8167 + r8168;
        double r8170 = r8169 / r8158;
        double r8171 = exp(r8170);
        double r8172 = r8149 + r8171;
        double r8173 = r8163 / r8172;
        double r8174 = r8162 + r8173;
        return r8174;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019179 
(FPCore (NdChar Ec Vef EDonor mu KbT NaChar Ev EAccept)
  :name "Bulmash initializePoisson"
  (+ (/ NdChar (+ 1.0 (exp (/ (- (- (- (- Ec Vef) EDonor) mu)) KbT)))) (/ NaChar (+ 1.0 (exp (/ (+ (+ (+ Ev Vef) EAccept) (- mu)) KbT))))))

Backtrace

result arity mismatch; expected number of values not received expected: 2 received: 1 in: local-binding form values...: 0		L	C
loop	/data/pavpan/nightlies/herbie/various-cleanup/src/core/extraction.rkt	25	2
(unnamed)	/data/pavpan/nightlies/herbie/various-cleanup/src/core/simplify.rkt	26	0
simplify!	/data/pavpan/nightlies/herbie/various-cleanup/src/mainloop.rkt	217	0
run-improve43	/data/pavpan/nightlies/herbie/various-cleanup/src/mainloop.rkt	339	0
(unnamed)	/opt/racket-7.0/collects/racket/private/more-scheme.rkt	261	28
run	/opt/racket-7.0/share/pkgs/profile-lib/main.rkt	39	2
profile-thunk16	/opt/racket-7.0/share/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/opt/racket-7.0/collects/racket/private/more-scheme.rkt	261	28