Brms: The correlation between endogenous variable and exogenous variable in Structural Equation Model

Interfaces brms
1

I want to use brms package to perform the structural equation model.

The generic model code used was:

a_mod<-bf(a~b+c+d+e+f+g)

b_mod<-bf(b~c+d+e)

c_mod<-bf(c~d)

I want to add the correlation between g and c (g~~c).

  1. When I add the reference in set_rescor(TRUE) in brms, like this,

mod1<-brms(a_mod+ b_mod+ c_mod+set_rescor(T))

I can get residual correlation among all endogenous variables, not the correlation between g and c. And the residual correlation and the correlation are not same, right?

  1. When I use the code,

mod2<-brms(bf(mdbing(g,c)~0+ a_mod+ b_mod +c_mod+ set_rescor(TRUE)))

It reminds me:

error: Don’t know how to add a mvbrmsformulabform object without the response variable name. See help(‘brmsformula-helpers’) for more details.

Is there a way to obtain the correlation between a specified endogenous variable and exogenous variable (i.e., the correlation between g and c ) OR specified endogenous variables ( i.e., the correlation between a and b )?