st: mle of panel data with fixed effect

[王武俊 <wwjgggs@pku.edu.cn>]

Hello. I want to estimate the model: y_it=x_it'*beta1+sqrt(exp(x_it'*beta2))*epislon_it+alfa_i. i stands for individual and t stands for time. epsilon_it~N(0,1), alfa_it~N(0,sigma). beta1 and beta2 are the parameters I want to get. First I sum the above equation within i and divide it by T.Then I use the above equation to subtract the the equation we just get to get an eqution without alfa_i.That is:  
y_it-summation(y_is)/T=x_it'*beta-summation(x_is'*beta1)/T+sqrt(exp(x_it'*beta2))*epislon_it-summation{sqrt(exp(x_is'*beta2))*epislon_is}/T   s=1,,T
And I drop the last observation of each i.So t=1,,,T-1. Below is the mle I have written:



program mlefe
version 11
		args todo b lnf
		tempvar theta1 theta2     //theta1=xb1,theta2=xb2
		mleval `theta1'=`b',eq(1)
		mleval `theta2'=`b',eq(2)
		local y "$ML_y1"
		tempvar u usum u1 a idnum tnum lnl last
		tempname ma mu1 diag t1 t2 t3 V omega inv uvu mat1
		
		
		
		sort $panel $year
		gen double `u'=`y'-`theta1'         
		by $panel: egen double `usum'=mean(`u')
		gen double `u1'=`u'-`usum'    //demean
		gen double `a'=exp(`theta2')
		
		sort $year $panel              // generate N
		by $year: gen `idnum'=_n
		sum `idnum'
		local N=r(max)           
		
		sort $panel $year           // generate T
		by $panel: gen `tnum'=_n 
		sum `tnum'
		local T=r(max)
		
		sort $panel $year
		
		gen double `lnl'=.
		local i=1
		while `i'<`N'+1 {
		    
		    mkmat `a' `u1' if `idnum'==`i',mat(`mat1')
			matrix `ma'=`mat1'[1...,1]
			matrix `mu1'=`mat1'[1..`T'-1,2]
			matrix `diag'=diag(`ma')
			matrix `t1'=J(1,`T',1)#`ma'
			matrix `t2'=-(`t1'+`t1'')/`T'
			matrix `t3'=J(`T',`T',1)#(J(1,`T',1)*`ma'/(`T'^2))
			matrix `V'=`diag'+`t2'+`t3'
			matrix `omega'=`V'[1..`T'-1,1..`T'-1]   //variance matix omega
			matrix `inv'=invsym(`omega')
			matrix `uvu'=`mu1''*`inv'*`mu1'
			matrix list `uvu'
			replace `lnl'=-0.5*(`T'-1)*ln(2*3.1415926)-0.5*ln(det(`omega'))-0.5*trace(`uvu') if `idnum'==`i'  //log-likelihood of the ith panel
			local i=`i'+1
			
		}
		by $panel: gen byte `last'=(_n==_N)
		mlsum `lnf'=`lnl' if `last'   // put log-likelihood of each panel on the last observation within the panel
end

clear
set more off
set obs 100
set seed 12345
gen x = invnormal(uniform())
gen id = 1 + floor((_n - 1)/10)
bys id: gen fe = invnormal(uniform())
bys id: replace fe = fe[1]
bys id: gen year=_n
gen y = 3*x+3 + fe + invnormal(uniform())*exp(0.3*x+0.3)^0.5  //generate simulation data
global panel="id"
global year="year"
ml model d0 mlefe (y =x) (x)
ml check
ml search
ml maximize


After the execution, the result is:

Test 1:  Calling mlefe to check if it computes log likelihood and
         does not alter coefficient vector...
         Passed.

Test 2:  Calling mlefe again to check if the same log likelihood value is
         returned...
         Passed.

Test 3:  Calling mlefe to check if 1st derivatives are computed...
         test not relevant for type d0 evaluators.

Test 4:  Calling mlefe again to check if the same 1st derivatives are
         returned...
         test not relevant for type d0 evaluators.

Test 5:  Calling mlefe to check if 2nd derivatives are computed...
         test not relevant for type d0 evaluators.

Test 6:  Calling mlefe again to check if the same 2nd derivatives are
         returned...
         test not relevant for type d0 evaluators.

------------------------------------------------------------------------------
Searching for alternate values for the coefficient vector to verify that mlefe
returns different results when fed a different coefficient vector:

Searching...
initial:       log likelihood =     -<inf>  (could not be evaluated)
searching for feasible values +

feasible:      log likelihood = -225.61221
improving initial values ......+...
improve:       log likelihood =  -225.0945

continuing with tests...
------------------------------------------------------------------------------

Test 7:  Calling mlefe to check log likelihood at the new values...
         FAILED; mlefe returned error 504.


Can anybody help me with it? Thank you in advance!
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/