I am trying to solve a set of coupled differential eqns., but getting the errors as mentioned in the title.

```
replace = {Subscript(m, (Phi)) -> 10^-5, (CapitalGamma) ->
10^-11, (Lambda) -> 0.01, (Xi) -> -1, m -> 10^-21, k -> 10^59,
Subscript(M, P) -> 1};
V(t_) := 1/2 Subscript(m, (Phi))^2 (Phi)(t)^2 /. replace ;
R(t_) :=
Subscript(M, P)^-2 (4 V(t) - (Phi)'(t)^2/(k^2 a(t)^2)) /. replace;
(Chi)i(t_) := ((- (Xi) R(t) - m^2)/(Lambda))^(1/2) /. replace;
eqna = (Phi)''(t) + 2 a'(t)/a(t) (Phi)'(t) +
k a(t) (CapitalGamma) (Phi)'(t) +
k^2 a(t)^2 D(V(t), (Phi)(t)) /. replace;
eqnb = (Chi)''(t) + 2 a'(t)/a(t) (Chi)'(t) +
k^2 a(t)^2 (Lambda) (Chi)(t)^3 + k^2 a(t)^2 m^2 (Chi)(t) +
k^2 a(t)^2 (Xi) R(t) (Chi)(t) /. replace;
eqnc = k Subscript((Rho), r)'(t)/a(t) +
4 k a'(t)/
a(t)^2 Subscript((Rho), r)(t) - (CapitalGamma) (Phi)'(t)^2/
a(t)^2 /. replace;
eqnd = a'(t)/a(t) - Sqrt(
1/(3 Subscript(M,
P)^2) (1/2 (Phi)'(t) ^2 + k^2 a(t)^2 V(t) +
k^2 a(t)^2 Subscript((Rho), r)(t))) /. replace;
sol1 = NDSolve({eqna == 0 , eqnb == 0, eqnc == 0,
eqnd == 0, (Phi)(-60 ) == 15 , (Phi)'(-60) ==
0, (Chi)(-60) == (Chi)i(-60), (Chi)'(-60) == 0,
Subscript((Rho), r)(-60) == 10^-20,
a(-60) == 1}, {(Phi), (Chi), Subscript((Rho), r), a}, {t, -60,
5}) // FullSimplify
```

Error:NDSolve::ndsz: At t == -60., step size is effectively zero; singularity or stiff system suspected.

I think other solutions to this problem ndsz : step size is effectively zero; singularity or stiff system suspected didn’t match with my problem.

Also, while plotting:

`Plot({Evaluate(Abs((Chi)(t)) /. sol1)}, {t, -60, 5}, PlotRange -> All, ImageSize -> Large, Frame -> True)`

error is appearing as :

InterpolatingFunction::dmval: Input value {-59.9987} lies outside the range of data in the interpolating function. Extrapolation will be used.

Note that -59.9 is inside the range.